This finishes the refactoring of unary math operation handling so that
IntermUnary::promote has the complete code for setting the return type of
the node.

BUG=angleproject:952
TEST=angle_unittests, WebGL conformance tests

Change-Id: I19bd8d53029e24f734c9436eceb446b37e7fcf26
Reviewed-on: https://chromium-review.googlesource.com/262416Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>