When initializing the cache state for a merge, we should never use
registers multiple times. Other code paths leading to the same merge
point might provide different values for the different slots there.

R=ahaas@chromium.org

Bug: v8:7035, v8:6600
Change-Id: I8e409b494af0fdc1a5045ec04571611b97fcaf86
Reviewed-on: https://chromium-review.googlesource.com/754816
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49150}