[GlobalISel] LegalizationArtifactCombiner: Fix a bug in tryCombineMerges

Like COPY instructions explained in D70616, we don't check the constraints
when combining G_UNMERGE_VALUES. Use the same logic used in D70616 to check
if registers can be replaced, or a COPY instruction needs to be built.

https://reviews.llvm.org/D70564