[llvm] [AMDGPU] Fix illegal AGPR reclassification in RewriteMFMAFormStage (PR #200972)

[Petr Kurapov via llvm-commits]

https://github.com/kurapov-peter approved this pull request.

A bit pessimistic, but looks like it's gonna do the trick.

https://github.com/llvm/llvm-project/pull/200972