i8x16.extract_lane_u is pextrb, and i16x8.extract_lane_u is pextrw, we
can merge them instead of having separate opcodes.

R=bbudge@chromium.org

Bug: v8:10975
Change-Id: I7793a795905157b6094b1470d3437988c982af91
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2481834Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70771}