This CL adds vector splat byte, word and halfword to
codegen, disassembler and the simulator.

It also optimizes a number of Simd opcodes by using
the added instructions as well as VSX splat immediate (xxspltib).

Change-Id: I2c4eba33e81542f901d7cdc669c50b510e48c4c8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2909525Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#74701}