This CL implements load_extend with 2 lanes and all load_splat
operations on IA32. The necessary assemblers together with their
corresponding disassemblers and tests are also added in this CL.
The newly added opcodes include: S8x16LoadSplat, S16x8LoadSplat,
S32x4LoadSplat, S64x2LoadSplat, I64x2Load32x2S, I64x2Load32x2U.

Bug: v8:9886
Change-Id: I0a5dae0a683985c14c433ba9d85acbd1cee6705f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1982989Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Zhiguo Zhou <zhiguo.zhou@intel.com>
Cr-Commit-Position: refs/heads/master@{#65937}