[llvm] [HashRecognize] Rewrite arePHIsIntertwined (PR #144878)

[Piotr Fusik via llvm-commits]

pfusik wrote:

Thank you for your last paragraph.

My concern is that we could miscompile this specially crafted code:

```
  %evil.xor = xor i8 %data, %crc.trunc
  %zero = some value that is known to be zero
  %evil.mul = mul i8, %evil.xor, %zero
  %xor.data.crc = xor i8 %evil.mul, %crc.trunc
  %and.data.crc = and i8 %xor.data.crc, 1
  %data.next = lshr i8 %data, 1
  %check.sb = icmp eq i8 %and.data.crc, 0
  %crc.lshr = lshr i16 %crc, 1
  %xor = xor i16 %crc.lshr, -24575
  %crc.next = select i1 %check.sb, i16 %crc.lshr, i16 %xor
```

`isConditionalOnXorOfPHIs` will match `%evil.xor`, CRC computation appears correct because of `%xor.data.crc`, the loop doesn't use `%data` for CRC computation yet we incorrectly report LHSAux.


https://github.com/llvm/llvm-project/pull/144878