1. Simplify code by using asserts instead of adding internal errors to
log.

2. Add a TIntermBinary constructor that takes left and right operand
nodes as parameters.

3. Remove TIntermediate functions with trivial functionality.

BUG=angleproject:952
TEST=angle_unittests

Change-Id: I2e0e52160c9377d8efcf15f14fd59f01cb41bd83
Reviewed-on: https://chromium-review.googlesource.com/372720
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>