If the destination register of a binop is the same register as the
right hand side, we would first move the left hand side into that
register (overwriting the value of the rhs), and then use the rhs.
This CL fixes this issue and adds a regression test.

R=ahaas@chromium.org

Bug: v8:6600, v8:7033
Change-Id: Ief90b5bcffc65823037bc57fb00741b2448e6375
Reviewed-on: https://chromium-review.googlesource.com/753462
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49137}