This change adds constant folding support for following derivative
functions:
    - dFdx, dFdy and fwidth.
As per spec, derivatives of constant arguments should be 0, so this
change just sets result components to zero when the above operations
are performed on constant argument.

BUG=angleproject:913
TEST=dEQP Tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.
fragment_processing.*
(All 12 tests started passing with this change.)

Change-Id: I67b7c5abd970b740e15e3bd1ee56721e81bbecc4
Reviewed-on: https://chromium-review.googlesource.com/283759Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>