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

[Giuseppe Rossini via llvm-commits]

giuseros wrote:

Merging this in. Canonicalize tests still contain some `undef` tests, but I had a chat with @arsenm  and these are acceptable (removing them would leave some `undef` case untested). Thanks for the review @arsenm !

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