[GlobalISel][Legalizer] Use ArtifactValueFinder first for unmerge combines before trying others.

This is motivated by an pathological compile time issue during unmerge combining.

We should be able to use the AVF to do simplification. However AMDGPU
has a lot of codegen changes which I'm not sure how to evaluate.

Differential Revision: https://reviews.llvm.org/D109748