The arm implementation made the assumption that the {lhs} and {dst}
registers are either the same, or there is no overlap. This assumption
does not hold.
ia32 on the other hand has a lot of complicated logic (and unnecessary
code generation) for different cases of overlap.

This CL fixes the arm issue *and* simplifies the ia32 logic by making
the arm assumption hold, and using it to eliminate special handling on
ia32.

R=thibaudm@chromium.org

Bug: chromium:1146861
Change-Id: I8753c2ed70349e735c03293130c899c0c8a3a671
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2526388Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71060}