This is similar to the optimization for f32x4 dup + mul in
https://crrev.com/c/2719083. Refactor the pattern-matching code into a
helper function that returns a struct with all the necessary fields to
emit the optimized fmul by element instruction.

Add similar unittests and a negative test as well.

Bug: v8:11257
Change-Id: I79ab0bc783f43397191a54bf6fa736dd4dc8d807
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2728428Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73164}