[llvm] [RegAlloc] Account for use availability when applying rematerializable weight discount (PR #159180)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM for the RISC-V tests and heuristic change.  Please wait for @arsenm to confirm, particularly the AMDGPU tests.  .  

https://github.com/llvm/llvm-project/pull/159180