When atomic operations are used in loops, return the correct opcode length
for loop assignment.

Bug=v8:6842,v8:6532

Change-Id: I306db704d8a0baa5d98c05702360e6dfae11cbfa
Reviewed-on: https://chromium-review.googlesource.com/699561Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48273}