This change adds constant folding support for vector relational
built-ins.

BUG=angleproject:913
TEST=dEQP Tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.vector_relational*
(Fixes all 138 tests)

Change-Id: I291e332f2afb3ce3d6596e634f509995dbf35164
Reviewed-on: https://chromium-review.googlesource.com/272344Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>