[llvm] [AMDGPU] Fix nondeterminism in SIFixSGPRCopies (PR #70644)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Can you add a test? I assume it would be a flaky test before this patch, but should pass every time after this patch.

OK I've force pushed (sorry) to demonstrate adding a test with the codegen I happened to get on my machine, and then the second commit shows how the codegen changes to something that hopefully should be the same for everyone.

https://github.com/llvm/llvm-project/pull/70644