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: I22af56876d1b7ce305697bf9bf43ad9ec5d8a3a5 Reviewed-on: https://chromium-review.googlesource.com/263708 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Olli Etuaho <oetuaho@nvidia.com>