[llvm] [AMDGPU] Precommit test for issue in amdgpu-rewrite-agpr-copy-mfma, (PR #168609)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Test is still really big, but it's very difficult to shrink these. Can at least fix a few of the weird reduction artifacts 

https://github.com/llvm/llvm-project/pull/168609