Yingwei Zheng b5f4210e9f [InstCombine] Drop nuw flag when CtlzOp is a sub nuw (#91776) ...

See the following case:
```
define i32 @src1(i32 %x) {
  %dec = sub nuw i32 -2, %x
  %ctlz = tail call i32 @llvm.ctlz.i32(i32 %dec, i1 false)
  %sub = sub nsw i32 32, %ctlz
  %shl = shl i32 1, %sub
  %ugt = icmp ult i32 %x, -2
  %sel = select i1 %ugt, i32 %shl, i32 1
  ret i32 %sel
}

define i32 @tgt1(i32 %x) {
  %dec = sub nuw i32 -2, %x
  %ctlz = tail call i32 @llvm.ctlz.i32(i32 %dec, i1 false)
  %sub = sub nsw i32 32, %ctlz
  %and = and i32 %sub, 31
  %shl = shl nuw i32 1, %and
  ret i32 %shl
}
```
`nuw` in `%dec` should be dropped after the select instruction is
eliminated.

Alive2: https://alive2.llvm.org/ce/z/7S9529

Fixes https://github.com/llvm/llvm-project/issues/91691.

2024-05-13 14:27:59 +08:00

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