[llvm] [AMDGPU] Fix vector legalization for bf16 valu ops (PR #158439)

[Matt Arsenault via llvm-commits]

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

LGTM with test nit. Test also seems missing the big FMA cases. Also follow up should try handling fsub and all the min/maxes 

https://github.com/llvm/llvm-project/pull/158439