[GlobalISel] Better verification of G_UNMERGE_VALUES

Verify three cases of G_UNMERGE_VALUES separately:

1. Splitting a vector into subvectors (the converse of
   G_CONCAT_VECTORS).
2. Splitting a vector into its elements (the converse of
   G_BUILD_VECTOR).
3. Splitting a scalar into smaller scalars (the converse of
   G_MERGE_VALUES).

Previously #1 allowed strange combinations like this:
  %1:_(<2 x s16>),%2:_(<2 x s16>) = G_UNMERGE_VALUES %0(<2 x s32>)
This has been tightened up to check that the source and destination
element types match, and some MIR test cases updated accordingly.

Differential Revision: https://reviews.llvm.org/D111132