Prototype load lane instructions on ARM Liftoff.

We had a helper function for load lane that was living in
instruction-selector. Move it out to assembler-arm so we can reuse that
in Liftoff.

Bug: v8:10975
Change-Id: Ic6e15c23eb778fb94a882609be622d2ca1f61ddb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2614225
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72106}