Currently AtomicStores use AtomicExchange to store to memory, but
AtomicExchange produces an output that is ignored by the AtomicStore
visitor, a side effect of this is that a register already in use gets
overwritten by the output of the exchange.

BUG:v8:7602

Change-Id: I4ec3107a0a27503611e349e6f56ca9492d05d9f8
Reviewed-on: https://chromium-review.googlesource.com/1134576Reviewed-by: Ben Smith <binji@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54565}