This fills out all possible combinations of SimdType conversions. The
implementation is not the best, it uses existing conversions instead of
creating new specific ones, but is sufficient, since we don't prioritize
performance for lowering.

I added if/else clauses to the existing implementation, so that the diff
is easier to read. I will put up a follow-up to convert it into switch
statements.

Bug: chromium:1134898,v8:10507
Change-Id: I5fcb3597882b0ea3084fdc79eb1dab9b8629044b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2453462
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70393}