[NFC][Regalloc] Split canEvictInterference into hint and general

There are 2 eviction queries. One is made by tryAssign, when it attempts to
free an interference occupying the hint of the candidate. The other is
during 'regular' interference resolution, where we scan over all
physical registers and try to see if we can evict live ranges in favor
of the candidate. We currently use the same logic in both cases, just
that the former never passes the cost to any subsequent query.
Technically, the 2 decisions could be implemented with different
policies.

This patch splits the 2.

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

Differential Revision: https://reviews.llvm.org/D114019