[llvm] [AMDGPU] Refresh RegisterClassInfo after reserving WWM registers (PR #221849)

[Michael Halkenhäuser via llvm-commits]

mhalk wrote:

Thanks for the heads-up @vvereschaka!
A fixing PR is ready for review, but presumably I'll go for a forward-fix + post-merge review.

https://github.com/llvm/llvm-project/pull/221849