[NFC][Regalloc] Factor out eviction decision from eviction attempt

This splits tryEvict into a const tryFindEvictionCandidate, which
attempts to find a candidate, and the actual eviction (should the former
be successful)

The newly introduced tryFindEvictionCandidate will move subsequently
into the RegAllocEvictionAdvisor.

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

Differential Revision: https://reviews.llvm.org/D113941