[llvm] AMDGPU: Fix treating divergent loads as uniform (PR #168785)

[Jay Foad via llvm-commits]

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jayfoad wrote:

Did you try changing this to explicitly check for a PseudoSourceValue instead of `!Ptr`? E.g.
```suggestion
// If Ptr is null, then that means this mem operand contains a
// PseudoSourceValue like GOT.
if (MMO->getPseudoValue())
  return true;
if (Ptr && isa<UndefValue, Constant, GlobalValue>(Ptr))
return true;
```

https://github.com/llvm/llvm-project/pull/168785