This makes sure that shift expressions (not wrapped in parentheses) can
appear as part of the index in a valid heap access expression. Only the
last operand of a sequence of shift expressions is taken into account
when validating the heap access.

R=jarin@chromium.org
TEST=mjsunit/regress/regress-6700
BUG=v8:6700,chromium:754751

Change-Id: Icc7a71bd64461da4d3daea41b995964e3dfc6dc6
Reviewed-on: https://chromium-review.googlesource.com/623811
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47497}