[llvm] [clang] [InstCombine] Infer zext nneg flag (PR #71534)

[via cfe-commits]

mikaelholmen wrote:

I think this patch causes miscompiles. Reproduce with
```opt bbi-88690.ll -passes=instcombine -S -o -```
So with this patch instcombine turns
```
@v_936 = global i16 -3276, align 1
@v_937 = global i24 0, align 1

define i16 @main() {
entry:
  %0 = load i16, ptr @v_936, align 1
  %unsclear = and i16 %0, 32767
  %resize = zext i16 %unsclear to i24
  %unsclear1 = and i24 %resize, 8388607
  store i24 %unsclear1, ptr @v_937, align 1
  ret i16 0
}
```
into
```
@v_936 = global i16 -3276, align 1
@v_937 = global i24 0, align 1

define i16 @main() {
entry:
  %0 = load i16, ptr @v_936, align 1
  %resize = zext nneg i16 %0 to i24
  store i24 %resize, ptr @v_937, align 1
  ret i16 0
}
```
I.e the and with 32767 (0x7fff) is gone and instead the zext got "nneg"?
But the value in v_936 can be, and actually _is_ negative.

[bbi-88690.ll.gz](https://github.com/llvm/llvm-project/files/13306009/bbi-88690.ll.gz)


https://github.com/llvm/llvm-project/pull/71534