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: I63133ee8c04c4a8d6cb30da5788e9227c05d4cbe
Reviewed-on: https://chromium-review.googlesource.com/266071Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>