Support constant folding of exponential built-ins This change adds constant folding support for unary exponential built-ins - exp, log, exp2, log2, sqrt and inversesqrt. BUG=angleproject:913 TEST= dEQP tests dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.exponential* (48 out of 56 tests started passing with this change) Change-Id: I4b98782c4c4b72dd7d60dfc4f18ba6961526ec41 Reviewed-on: https://chromium-review.googlesource.com/266797 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>