AMDGPU/GlobalISel: Rework legalization for extract/insert vector elt

Use G_MERGE_VALUES and G_UNMERGE_VALUES on vector elements instead of
G_EXTRACT and G_INSERT when doing custom legalization for
G_EXTRACT_VECTOR_ELT and G_INSERT_VECTOR_ELT.
With this approach legalization artifact combiner gets direct access
to all vector elements.

Differential Revision: https://reviews.llvm.org/D116115