Intermediate should only have logic for creating node objects, validation
of parameter types belongs in ParseContext.

BUG=angleproject:952
TEST=angle_unittests, WebGL conformance tests

Change-Id: Ie90697641fabb2a837ccc4571a93616d63ea64e6
Reviewed-on: https://chromium-review.googlesource.com/262414Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>