[llvm] [AMDGPU][Scheduler] Fix incorrect region index in EXPENSIVE_CHECKS (PR #179461)

[Mikael Holmen via llvm-commits]

mikaelholmen wrote:

I've verified that this solves the EXPENSIVE_CHECKS problems I reported in
 https://github.com/llvm/llvm-project/pull/177206#issuecomment-3840310349
but I think you should get this reviewed by AMD folks.

https://github.com/llvm/llvm-project/pull/179461