This is needed to make way for further AST transformations to handle array
expressions that need to work correctly together with unfolding short-
circuiting operators. This also improves the maintainability of HLSL output
by isolating the unfolding into a separate compilation step.

The new version of UnfoldShortCircuit traverser will traverse the tree until
an expression that needs to be unfolded is encountered. It then unfolds it and
gets reset. The traverser will be run repeatedly until no more operations to
unfold are found. This helps with keeping the traverser's design relatively
simple.

All declarations are separated to single declarations before short-circuit
unfolding is run. Previously OutputHLSL already output every declaration
separately.

BUG=angleproject:960
TEST=WebGL conformance tests, angle_unittests, angle_end2end_tests

Change-Id: Id769be396adbd4c0223e418980dc464dd855f019
Reviewed-on: https://chromium-review.googlesource.com/270460Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>