This makes sure that an existing unmodified snapshot works together with
the --branch-load-poisoning feature by unconditionally resetting the
underlying register in {CEntryStub::Generate}.

R=mvstanton@chromium.org

Change-Id: I4d9f12a3b45bd7b46eda55a342beadf9b9a4bb8a
Reviewed-on: https://chromium-review.googlesource.com/926365
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51397}