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: I5d3e96ef2062116cbf164a7a42cba096bd4d5389
Reviewed-on: https://chromium-review.googlesource.com/269317Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>