This change adds constant folding support for following geometric
built-ins:
    - length, distance, dot, cross, normalize, faceforward,
      reflect and refract.

BUG=angleproject:913
TEST=angle_unittests, dEQP Tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.geometric.*
(56 tests started passing with this change)

Change-Id: I973689554bb8c30ee3ebdf71f1b8a051ceb5e0cf
Reviewed-on: https://chromium-review.googlesource.com/273097Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>