[GlobalISel] Combine scalar unmerge(trunc)

Summary:
Combine unmerge(trunc) to enable other merge combines.
Without this combine, the scalar unmerge(trunc(merge))
pattern cannot be combined and easily lead to
hard-to-legalize merge/unmerge artifacts.

Reviewed By: arsenm

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79567