Unfolding of short-circuiting operators (ternary and logical operators) may
create if statements in the global scope, which is not valid HLSL. Use existing
deferred global initialization function to defer execution of if statements in
the global scope.

TEST=WebGL conformance tests
BUG=angleproject:819

Change-Id: I2b0afcc6824dab6bb87eb6abed609e75b1384dab
Reviewed-on: https://chromium-review.googlesource.com/270461Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>