[GlobalISel][Legalizer] Look thro copies while combining G_UNMERGE's

As we're becoming stricter w/ respect to not allowing vregs having LLTs
and regclasses assigned both mid-globalisel pipeline, the number of
extra copies grows, some of which separate G_UNMERGE's from their
corresponding G_MERGE's, becoming a performance concern.

It's worth mentioning that we're already looking through copies while
combining legalization artifacts for every kind of artifact but
G_UNMERGE.

Reviewed By: aditya_nandakumar

Reviewers: ab, t.p.northover, volkan, javed.absar

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D45644


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330660 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed