Integer splats (especially for sizes < 32-bits) does not directly
translate to a single instruction on x64. We can do better for special
values, like 0, which can be lowered to `xor dst dst`. We do this check
in the instruction selector, and emit a special opcode kX64S128Zero.

Also change the xor operation for kX64S128Zero from xorps to pxor. This
can help reduce any potential data bypass delay (search for this on
agner's microarchitecture manual for more details.). Since integer
splats are likely to be followed by integer ops, we should remain in the
integer domain, thus use pxor.

For i64x2.splat the codegen goes from:

  xorl rdi,rdi
  vmovq xmm0,rdi
  vmovddup xmm0,xmm0

to:

  vpxor xmm0,xmm0,xmm0

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

Bug: v8:11093
Change-Id: I26b092032b6e672f1d5d26e35d79578ebe591cfe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2516299Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70977}