[AMDGPU] Extend test coverage for cross RC register coalescing (llvm#138617)

[[Change-Id:
I36894fc36e6e6214930fae67f2ca35999abf3b88](llvm#132137)
](llvm#132137) landed, but was
breaking bots. In the rebase, I accidentally brought in unintended
changes. I have reverted the commit
llvm@8b9ae65
and fixed the rebase.

This PR is to land the original patch / extension to the test.

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Co-authored-by: Matt Arsenault <[email protected]>

Author: jrbyrnes