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/266797Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>