This fixes a regression from recent refactoring of unfolding short-circuiting
operators. mUsesDiscardRewriting used to be set based on detecting patterns
that the AST traverser now overwrites before the code reaches OutputHLSL. With
this patch mUsesDiscardRewriting is set more conservatively, which ensures that
using discard works correctly also in case the input shader contains the same
kind of pattern as the short-circuiting operator unfolding creates.

TEST=WebGL conformance tests on D3D9
BUG=chromium:488552

Change-Id: Iea0103822a268079593b0913f0b6fc54c745c6e3
Reviewed-on: https://chromium-review.googlesource.com/271761Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>