Also piece together an addAssign function in ParseContext that uses the
binary op array check. This will make it easier to change the
array-related checks in the future to use shaderVersion. Moving
validation out from IntermBinary::promote also makes the architecture
clearer, promote()'s role should be mainly to determine the type of the
return value of the binary operation, not to do validation.

BUG=angleproject:941
TEST=angle_unittests, WebGL conformance tests

Change-Id: If1de33ea250893527be7f0d7930d4678a0864684
Reviewed-on: https://chromium-review.googlesource.com/260571Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>