Integer splats (especially for sizes < 32-bits) does not directly
translate to a single instruction on ia32. We can do better for special
values, like 0, which can be lowered to `eor dst dst`. We do this check
in the instruction selector, and emit a special opcode kX64S128Zero.

Also add a unittest to verify this optimization, and necessary
raw-assembler methods for the test.

Bug: v8:11093
Change-Id: Icfebef06a5ecf49619ea54f31a5296094fb53ff2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2516300Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71024}