Add constant folding support for min,max and clamp This change adds necessary mechanism to support constant folding of built-ins that take more than one parameter and also adds constant folding support for min, max and clamp built-ins. BUG=angleproject:913 TESTS=dEQP tests (126 tests started passing with this change) dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common.min_* dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common.max_* dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common.clamp_* Change-Id: Iccc9bf503a536f2e3c144627e64572f2f95db9db Reviewed-on: https://chromium-review.googlesource.com/271251 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>