Moving a register to itself is not only unnecessary overhead, it also
breaks invariants in the StackTransferRecipe.

R=ahaas@chromium.org

Bug: v8:6600, chromium:793551
Change-Id: I659fd66b4f2d4564c437ed9fb048322af4299d97
Reviewed-on: https://chromium-review.googlesource.com/819231Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49992}