[llvm] [clang] [InstCombine] Infer disjoint flag on Or instructions. (PR #72912)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

Here's a simple-ish example:
```llvm
; Transforms/InstCombine/add.ll

define i5 @zext_sext_not(i4 %x) {
  %zx = zext i4 %x to i5
  %notx = xor i4 %x, 15
  %snotx = sext i4 %notx to i5
  %r = add i5 %zx, %snotx
  ret i5 %r
}
=>
define i5 @zext_sext_not(i4 %x) {
  %zx = zext i4 %x to i5
  %notx = xor i4 %x, 15
  %snotx = sext i4 %notx to i5
  %r = or disjoint i5 %zx, %snotx
  ret i5 %r
}
Transformation doesn't verify! (unsound)
ERROR: Target is more poisonous than source

Example:
i4 %x = undef

Source:
i5 %zx = #x00 (0)	[based on undef value]
i4 %notx = #xf (15, -1)	[based on undef value]
i5 %snotx = #x1f (31, -1)
i5 %r = #x1f (31, -1)

Target:
i5 %zx = #x0f (15)
i4 %notx = #xf (15, -1)
i5 %snotx = #x1f (31, -1)
i5 %r = poison
Source value: #x1f (31, -1)
Target value: poison
```

Essentially the code doesn't take into consideration that a same register may have different values when it is undef.

https://github.com/llvm/llvm-project/pull/72912