[NFC][Regalloc] Factor types that would be used by the eviction advisor

This is in prepartion of pulling the eviction decision-making into an
analysis pass, which would then allow swapping that decision making
process.

RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-November/153639.html

Differential Revision: https://reviews.llvm.org/D113929