This makes sure that function variables as well as function table
variables are properly typed as immutable, hence assignments to them
should cause validation failures.

R=clemensh@chromium.org
TEST=mjsunit/asm/immutable
BUG=chromium:721271

Change-Id: Ia3f65fd0782ca571ffcf99520fdbd8fc5a359d16
Reviewed-on: https://chromium-review.googlesource.com/503209Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45256}