This change adds const folding support for below mentioned common
built-ins:
    - isnan, ininf, floatBitsToInt, floatBitsToUint,
      intBitsToFloat and uintBitsToFloat

BUG=angleproject:913
TEST=angle_unittests(new: MathUtilTest.isNan/inInf), dEQP Tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common.*
(20 more tests started passing with this change)

Change-Id: Ifdedb251cd8b183b4999314c0f5de857bc20702f
Reviewed-on: https://chromium-review.googlesource.com/283767Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>