[GlobalISel] combine not symmetric merge/unmerge nodes.

Summary:
In some cases legalization ends up with not symmetric merge/unmerge nodes.
Transform it to merge/unmerge nodes.

Reviewers: t.p.northover, qcolombet, zvi

Reviewed By: t.p.northover

Subscribers: rovka, kristof.beyls, guyblank, llvm-commits

Differential Revision: https://reviews.llvm.org/D33626

llvm-svn: 305783