[GlobalISel] Enable artifact combiner to combine starting from a G_MERGE_VALUES.

We generally only combine starting from users to defs in the artifact combiner,
but this doesn't catch cases where at the point of combining a G_UNMERGE we don't
yet have the opposite G_MERGE on input yet since we haven't legalized that far.

This change adds the users of a G_MERGE to the artifact combiner worklist if one
of the uses is a G_UNMERGE or G_TRUNC.

Differential Revision: https://reviews.llvm.org/D77931