[GlobalISel] Add scalar widening for G_MERGE_VALUES destination

This adds support for the case where

WideSize = DstSize + K * SrcSize

In this case, we can pad the G_MERGE_VALUES instruction with K extra undef
values with width SrcSize. Then the destination can be handled via
widenScalarDst.

Differential Revision: https://reviews.llvm.org/D106814