[llvm] [AMDGPU] Extend test coverage for cross RC register coalescing (PR #132137)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

Force push for rebase -- This test wasn't passing with llvm-lit, and the failure wasn't resolved by `update_mir_test_checks`. I needed to manually rewrite the test to get it to pass 

https://github.com/llvm/llvm-project/pull/132137