Registers cannot be used as a merge destination if they have more than
one use, otherwise the merge will unexpectedly affect other uses of that
register.

R=ahaas@chromium.org,clemensb@chromium.org

Bug: chromium:1084151
Change-Id: I0d6ad97c585920357a37d95361e0320d32c71f4b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208851Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67904}