TIntermBinary::promote now has a single purpose of determining the
type resulting from a binary math operation. The TIntermBinary
constructor taking the left and right nodes can now also call promote
automatically, and promote is made into a private member of
TIntermBinary. Validation of binary math operand types is done
inside ParseContext.

BUG=angleproject:952
TEST=angle_unittests

Change-Id: I52a409f680c8d4120b757193972d03aed34c6895
Reviewed-on: https://chromium-review.googlesource.com/372624Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>