We lower each op into num_lanes loads, and connecting up the effects in
a chain.

s64x2 is not implemented since we lowering for 64x2 generally doesn't
work anyway.

Load extends are a bit more complicated, so we'll do that in a separate
change.

Bug: v8:9886
Change-Id: I80096827bf8e8e0db1ef0ad1b76759ed1797ca5e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2031893
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66183}