For splats, we can make use of vshufps to avoid a movss. Without
AVX, specific dst to be same as src in the instruction selector.

For extract lane, we can use vshufps to extract a float into a dst xmm,
and leave junk in the higher bits.

On the meshopt_decoder.js benchmark in linked bug, it removes about 7
movss instructions that did nothing. Hardware can do register renaming,
but let's not rely on that :)

R=bbudge@chromium.org

Bug: v8:10116
Change-Id: I4d68c10536a79659de673060d537d58113308477
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2481473
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70628}