[llvm] [AMDGPU] Allow rematerialization of instructions with virtual register uses (PR #124327)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

PSDB was fine except for the `assert(LM.none())`

I also modified the patch to allow for more rematerializations (e.g. not only for high pressure regions, always do the remats even if it doesn't help, etc), and there were no issues with PSDB.

https://github.com/llvm/llvm-project/pull/124327