[CVP} Replace SExt with ZExt if the input is known-non-negative

[CVP} Replace SExt with ZExt if the input is known-non-negative

Summary:
zero-extension is far more friendly for further analysis.
While this doesn't directly help with the shift-by-signext problem, this is not unrelated.

This has the following effect on test-suite (numbers collected after the finish of middle-end module pass manager):
| Statistic                            |     old |     new | delta | percent change |
| correlated-value-propagation.NumSExt |       0 |    6026 |  6026 |   +100.00%     |
| instcount.NumAddInst                 |  272860 |  271283 | -1577 |     -0.58%     |
| instcount.NumAllocaInst              |   27227 |   27226 | -1    |      0.00%     |
| instcount.NumAndInst                 |   63502 |   63320 | -182  |     -0.29%     |
| instcount.NumAShrInst                |   13498 |   13407 | -91   |     -0.67%     |
| instcount.NumAtomicCmpXchgInst       |    1159 |    1159 |  0    |      0.00%     |
| instcount.NumAtomicRMWInst           |    5036 |    5036 |  0    |      0.00%     |
| instcount.NumBitCastInst             |  672482 |  672353 | -129  |     -0.02%     |
| instcount.NumBrInst                  |  702768 |  702195 | -573  |     -0.08%     |
| instcount.NumCallInst                |  518285 |  518205 | -80   |     -0.02%     |
| instcount.NumExtractElementInst      |   18481 |   18482 |  1    |      0.01%     |
| instcount.NumExtractValueInst        |   18290 |   18288 | -2    |     -0.01%     |
| instcount.NumFAddInst                |  139035 |  138963 | -72   |     -0.05%     |
| instcount.NumFCmpInst                |   10358 |   10348 | -10   |     -0.10%     |
| instcount.NumFDivInst                |   30310 |   30302 | -8    |     -0.03%     |
| instcount.NumFenceInst               |     387 |     387 |  0    |      0.00%     |
| instcount.NumFMulInst                |   93873 |   93806 | -67   |     -0.07%     |
| instcount.NumFPExtInst               |    7148 |    7144 | -4    |     -0.06%     |
| instcount.NumFPToSIInst              |    2823 |    2838 |  15   |      0.53%     |
| instcount.NumFPToUIInst              |    1251 |    1251 |  0    |      0.00%     |
| instcount.NumFPTruncInst             |    2195 |    2191 | -4    |     -0.18%     |
| instcount.NumFSubInst                |   92109 |   92103 | -6    |     -0.01%     |
| instcount.NumGetElementPtrInst       | 1221423 | 1219157 | -2266 |     -0.19%     |
| instcount.NumICmpInst                |  479140 |  478929 | -211  |     -0.04%     |
| instcount.NumIndirectBrInst          |       2 |       2 |  0    |      0.00%     |
| instcount.NumInsertElementInst       |   66089 |   66094 |  5    |      0.01%     |
| instcount.NumInsertValueInst         |    2032 |    2030 | -2    |     -0.10%     |
| instcount.NumIntToPtrInst            |   19641 |   19641 |  0    |      0.00%     |
| instcount.NumInvokeInst              |   21789 |   21788 | -1    |      0.00%     |
| instcount.NumLandingPadInst          |   12051 |   12051 |  0    |      0.00%     |
| instcount.NumLoadInst                |  880079 |  878673 | -1406 |     -0.16%     |
| instcount.NumLShrInst                |   25919 |   25921 |  2    |      0.01%     |
| instcount.NumMulInst                 |   42416 |   42417 |  1    |      0.00%     |
| instcount.NumOrInst                  |  100826 |  100576 | -250  |     -0.25%     |
| instcount.NumPHIInst                 |  315118 |  314092 | -1026 |     -0.33%     |
| instcount.NumPtrToIntInst            |   15933 |   15939 |  6    |      0.04%     |
| instcount.NumResumeInst              |    2156 |    2156 |  0    |      0.00%     |
| instcount.NumRetInst                 |   84485 |   84484 | -1    |      0.00%     |
| instcount.NumSDivInst                |    8599 |    8597 | -2    |     -0.02%     |
| instcount.NumSelectInst              |   45577 |   45913 |  336  |      0.74%     |
| instcount.NumSExtInst                |   84026 |   78278 | -5748 |     -6.84%     |
| instcount.NumShlInst                 |   39796 |   39726 | -70   |     -0.18%     |
| instcount.NumShuffleVectorInst       |  100272 |  100292 |  20   |      0.02%     |
| instcount.NumSIToFPInst              |   29131 |   29113 | -18   |     -0.06%     |
| instcount.NumSRemInst                |    1543 |    1543 |  0    |      0.00%     |
| instcount.NumStoreInst               |  805394 |  804351 | -1043 |     -0.13%     |
| instcount.NumSubInst                 |   61337 |   61414 |  77   |      0.13%     |
| instcount.NumSwitchInst              |    8527 |    8524 | -3    |     -0.04%     |
| instcount.NumTruncInst               |   60523 |   60484 | -39   |     -0.06%     |
| instcount.NumUDivInst                |    2381 |    2381 |  0    |      0.00%     |
| instcount.NumUIToFPInst              |    5549 |    5549 |  0    |      0.00%     |
| instcount.NumUnreachableInst         |    9855 |    9855 |  0    |      0.00%     |
| instcount.NumURemInst                |    1305 |    1305 |  0    |      0.00%     |
| instcount.NumXorInst                 |   10230 |   10081 | -149  |     -1.46%     |
| instcount.NumZExtInst                |   60353 |   66840 |  6487 |     10.75%     |
| instcount.TotalBlocks                |  829582 |  829004 | -578  |     -0.07%     |
| instcount.TotalFuncs                 |   83818 |   83817 | -1    |      0.00%     |
| instcount.TotalInsts                 | 7316574 | 7308483 | -8091 |     -0.11%     |

TLDR: we produce -0.11% less instructions, -6.84% less `sext`, +10.75% more `zext`.
To be noted, clearly, not all new `zext`'s are produced by this fold.

(And now i guess it might have been interesting to measure this for D68103 :S)

Reviewers: nikic, spatel, reames, dberlin

Reviewed By: nikic

Subscribers: hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68654

llvm-svn: 374112