Shuffle the code around so that each part has a clear responsibility:
IntermUnary::promote is responsible for setting the return type of the
node, Intermediate::addUnaryMath is responsible for creating the node
object, and ParseContext::createUnaryMath is responsible for validating
the operand type.

This removes duplicated bool type check for logical not.

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

Change-Id: I9f5a0abb6434ad2730441ea9199ec3f5382ebcda
Reviewed-on: https://chromium-review.googlesource.com/262415Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>