[amdgpu] Fix broken error detection in LDS lowering

std::optional<uint32_t> can be compared to uint32_t without warning, but does
not compare to the value within the optional. It needs to be prefixed *.
Wconversion does not warn about this.
```
bool bug(uint32_t Offset, std::optional<uint32_t> Expect)
{
  return (Offset != Expect);
}
bool deref(uint32_t Offset, std::optional<uint32_t> Expect)
{
  return (Offset != *Expect);
}
```
Both compile without warnings. Wrote the former, intended the latter.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D146775