[AArch64][AMDGPU][GlobalISel] Remove vector handling from unmerge_dead_to_trunc (#82224)

This combine transforms an unmerge where only the first element is used
into a truncate. That works OK for scalar but for vector needs to insert
a bitcast to integers, perform the truncate then bitcast back to
vectors. This generates more awkward code than using an Unmerge.