Constant fold vector relational built-ins 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/272344 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>