Add checks that opaque operands can only be used with array indexing
and field section, as mentioned in ESSL 3.10 section 4.1.7.

BUG=angleproject:2028

Change-Id: I41b7f10785bf712dfc999f85ebff925341c51911
Reviewed-on: https://chromium-review.googlesource.com/497767
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>