This improves separation of responsibilities in the code: ParseContext
should handle operand type validation, while TIntermBinary::promote
should ideally only determine the type of the node based on the
operation and operands.

BUG=angleproject:952
TEST=angle_unittests

Change-Id: I9a8d8ede21cdf35de631623a62194c0da5c604d2
Reviewed-on: https://chromium-review.googlesource.com/372622
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>