[llvm] [AMDGPU] merge 16bit mov pairs in post-RA peephole (PR #208625)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

LGTM thanks, just some minor nits.

Have you done any correctness testing e.g. with Vulkan CTS?

https://github.com/llvm/llvm-project/pull/208625