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

[via llvm-commits]

dyung wrote:

My bot that builds without asserts is slower, but it did break: https://lab.llvm.org/buildbot/#/builders/202/builds/5425

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