[llvm] [AMDGPU][Scheduler] Fix inconsistent MI slots after rematerialization revert (PR #180646)

[Lucas Ramirez via llvm-commits]

lucas-rami wrote:

> Yeah, did none of the buildbots break?

Doesn't look like it (the buildbot failure above looks unrelated to the change).

> `Unknown command line argument '-debug-only=machine-scheduler'.`

I guess the flag requires that LLVM is built with assert? I didn't know that, I should probably just add `# REQUIRES: asserts` to the test. Apologies for the oversight. 

https://github.com/llvm/llvm-project/pull/180646