Next SSCA mitigation: Mask the function index on indirect calls. This
avoids speculative jumps to arbitrary memory.

R=titzer@chromium.org

Bug: v8:6600, chromium:798964
Change-Id: Id4a54fbb42096655d48965b63202bb58f98dc9aa
Reviewed-on: https://chromium-review.googlesource.com/1049627
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53076}