This change ensures that GVN does not move StringSubstring out of
switches, which might introduce partial redundancies.

Bug: chromium:816522
Change-Id: I63b91edd995c84b68d756ed5de08fa13567f3d80
Reviewed-on: https://chromium-review.googlesource.com/939621Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51610}