Load splat implementation is almost the same, except for the vector
format used for the output register. We encode this information in
MiscField (the size of each lane), and with some helper functions we can
easily reuse a single opcode for 4 load splats.

Bug: v8:10930
Change-Id: Ieed4dc7358821a0d1d7bab4add7a59d808c5aad8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2422354
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70098}