[InstCombine] Fold `cttz(lshr(-1, x) + 1)` to `width - x` (#91244)

Fold
``` llvm
define i64 @src(i64 %50) {
  %52 = lshr i64 -1, %50
  %53 = add i64 %52, 1
  %54 = call i64 @llvm.cttz.i64(i64 %53, i1 false)
  ret i64 %54
}
```
to
``` llvm
define i64 @tgt(i64 %50) {
  %52 = sub i64 64, %50
  ret i64 %52
}
```

as
https://github.com/llvm/llvm-project/pull/91171#pullrequestreview-2040663002
pointed out.

Alive2 proof: https://alive2.llvm.org/ce/z/2aHfYa

Note: the `ctlz` version of this pattern seems not exist in dtcxzyw's
benchmark, so put it aside for now.

0 files changed, 0 insertions, 0 deletions