[InstCombine] Fold conditional sign-extend of high-bit-extract into high-bit-extract...

[InstCombine] Fold conditional sign-extend of high-bit-extract into high-bit-extract-with-signext (PR42389)

This can come up in Bit Stream abstractions.

The pattern looks big/scary, but it can't be simplified any further.
It only is so simple because a number of my preparatory folds had
happened already (shift amount reassociation / shift amount
reassociation in bit test, sign bit test detection).

Highlights:
* There are two main flavors: https://rise4fun.com/Alive/zWi
  The difference is add vs. sub, and left-shift of -1 vs. 1
* Since we only change the shift opcode,
  we can preserve the exact-ness: https://rise4fun.com/Alive/4u4
* There can be truncation after high-bit-extraction:
  https://rise4fun.com/Alive/slHc1   (the main pattern i'm after!)
  Which means that we need to ignore zext of shift amounts and of NBits.
* The sign-extending magic can be extended itself (in add pattern
  via sext, in sub pattern via zext. not the other way around!)
  https://rise4fun.com/Alive/NhG
  (or those sext/zext can be sinked into `select`!)
  Which again means we should pay attention when matching NBits.
* We can have both truncation of extraction and widening of magic:
  https://rise4fun.com/Alive/XTw
  In other words, i don't believe we need to have any checks on
  bitwidths of any of these constructs.

This is worsened in general by the fact that we may have `sext` instead
of `zext` for shift amounts, and we don't yet canonicalize to `zext`,
although we should. I have not done anything about that here.

Also, we really should have something to weed out `sub` like these,
by folding them into `add` variant.

https://bugs.llvm.org/show_bug.cgi?id=42389

llvm-svn: 373964