AMDGPU/GlobalISel: Fix some crashs in g_unmerge_values/g_merge_values

This was crashing in the predicate function assuming the value
is a vector.

Copy more of what AArch64 uses. This probably needs more refinement
later, but I don't exactly understand what it means in some cases,
particularly since any legalization for these seems to be missing.

llvm-svn: 351693