[llvm] [AMDGPU] Handle negated f16 and fp conversion DAG combines (PR #213202)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Can you rebase this so the first commit in the PR adds the tests generated with the baseline checks, and then the diff that demonstrates the patch does anything is in the second commit 

https://github.com/llvm/llvm-project/pull/213202