[LCSSA] Skip blocks in sub-loops when scanning for uses.

[LCSSA] Skip blocks in sub-loops when scanning for uses.

Summary:
Scanning blocks in sub-loops for uses is unnecessary, as they were
already handled while dealing with the containing sub-loop.

This speeds up LCSSA for highly nested loops. For the test case in PR37202, it
halves the time spent in LCSSA. In cases were we won't be able to skip
any blocks, the additional lookup should be negligible.

Time-passes without this patch for test case from PR37202:

  Total Execution Time: 48.5505 seconds (48.5511 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  10.0822 ( 21.0%)   0.1406 ( 27.0%)  10.2228 ( 21.1%)  10.2228 ( 21.1%)  Loop-Closed SSA Form Pass
  10.0417 ( 20.9%)   0.1467 ( 28.2%)  10.1884 ( 21.0%)  10.1890 ( 21.0%)  Loop-Closed SSA Form Pass #2
   4.2703 (  8.9%)   0.0040 (  0.8%)   4.2742 (  8.8%)   4.2742 (  8.8%)  Unswitch loops
   2.7376 (  5.7%)   0.0229 (  4.4%)   2.7605 (  5.7%)   2.7611 (  5.7%)  Loop-Closed SSA Form Pass #5
   2.7332 (  5.7%)   0.0214 (  4.1%)   2.7546 (  5.7%)   2.7546 (  5.7%)  Loop-Closed SSA Form Pass #3
   2.7088 (  5.6%)   0.0230 (  4.4%)   2.7319 (  5.6%)   2.7324 (  5.6%)  Loop-Closed SSA Form Pass #4
   2.6855 (  5.6%)   0.0236 (  4.5%)   2.7091 (  5.6%)   2.7090 (  5.6%)  Loop-Closed SSA Form Pass #6
   2.1648 (  4.5%)   0.0018 (  0.4%)   2.1666 (  4.5%)   2.1664 (  4.5%)  Unroll loops
   1.8371 (  3.8%)   0.0009 (  0.2%)   1.8379 (  3.8%)   1.8380 (  3.8%)  Value Propagation
   1.8149 (  3.8%)   0.0021 (  0.4%)   1.8170 (  3.7%)   1.8169 (  3.7%)  Loop Invariant Code Motion
   1.6755 (  3.5%)   0.0226 (  4.3%)   1.6981 (  3.5%)   1.6980 (  3.5%)  Loop-Closed SSA Form Pass #7

Time-passes with this patch

  Total Execution Time: 29.9285 seconds (29.9276 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   5.2786 ( 17.7%)   0.0021 (  1.2%)   5.2806 ( 17.6%)   5.2808 ( 17.6%)  Unswitch loops
   4.3739 ( 14.7%)   0.0303 ( 18.1%)   4.4042 ( 14.7%)   4.4042 ( 14.7%)  Loop-Closed SSA Form Pass
   4.2658 ( 14.3%)   0.0192 ( 11.5%)   4.2850 ( 14.3%)   4.2851 ( 14.3%)  Loop-Closed SSA Form Pass #2
   2.2307 (  7.5%)   0.0013 (  0.8%)   2.2320 (  7.5%)   2.2318 (  7.5%)  Loop Invariant Code Motion
   2.0888 (  7.0%)   0.0012 (  0.7%)   2.0900 (  7.0%)   2.0897 (  7.0%)  Unroll loops
   1.6761 (  5.6%)   0.0013 (  0.8%)   1.6774 (  5.6%)   1.6774 (  5.6%)  Value Propagation
   1.3686 (  4.6%)   0.0029 (  1.8%)   1.3716 (  4.6%)   1.3714 (  4.6%)  Induction Variable Simplification
   1.1457 (  3.8%)   0.0010 (  0.6%)   1.1468 (  3.8%)   1.1468 (  3.8%)  Loop-Closed SSA Form Pass #4
   1.1384 (  3.8%)   0.0005 (  0.3%)   1.1389 (  3.8%)   1.1389 (  3.8%)  Loop-Closed SSA Form Pass #6
   1.1360 (  3.8%)   0.0027 (  1.6%)   1.1387 (  3.8%)   1.1387 (  3.8%)  Loop-Closed SSA Form Pass #5
   1.1331 (  3.8%)   0.0010 (  0.6%)   1.1341 (  3.8%)   1.1340 (  3.8%)  Loop-Closed SSA Form Pass #3

Reviewers: davide, efriedma, mzolotukhin

Reviewed By: davide, efriedma

Subscribers: hiraditya, dmgreen, llvm-commits

Differential Revision: https://reviews.llvm.org/D56848

llvm-svn: 351567