[llvm] [AMDGPU] Add a mir test to verify register-coalesce works for eliminate-save-restore pattern (PR #191583)

[Zeng Wu via llvm-commits]

================

----------------
zwu-2025 wrote:

Some of comments are folded so they are skipped. Sorry about that, and now all comments should be checked.

https://github.com/llvm/llvm-project/pull/191583