Move the simd post mvp flag check into caller, instead of only checking
in the default case. This allows us to check *all* opcodes to see if
they are post-mvp or not. Before this change, we missed the check for
load and store lanes.

Bug: v8:10975,chromium:1149078
Change-Id: Id3cff8169ef5137f54a49d28291698139116e8a0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2543170Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71272}