Move lhs into dst when lhs == rhs.

Bug: v8:6600, chromium:910223
Change-Id: I5bcb01b57705c52f363a43b2036f0ade0fd451e7
Reviewed-on: https://chromium-review.googlesource.com/c/1357041
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57996}