[llvm] [AMDGPU][GlobalISel] Guard against use of disallowed sub-registers (PR #188781)

[Jay Foad via llvm-commits]

jayfoad wrote:

The AArch64 cases are like:
```
  %41:gpr32all = COPY %42.sub_32:gpr32all
```
This is a weird degenerate case, trying to refer to a 32-bit subreg of a register that is already 32-bit. This is surely unintentional and should not be allowed. Could you try fixing this in the AArch64 backend please?

https://github.com/llvm/llvm-project/pull/188781