Previously, creating nodes for initializing each single array element
could result in memory bloat during translation when dealing with
large arrays. The resulting shader could also end up very long.
Initialize most arrays using a simple for loop instead. The loop is
compatible with ESSL 1.00 Appendix A limitations.

An exception is made for fragment outputs, so that they are not
indexed by non-constant values.

On some platforms using the a loop to initialize variables can cause
problems, so we also have a compiler flag for turning this behavior
off. The flag was already added earlier for a staggered rollout of
this functionality.

BUG=chromium:735497
TEST=angle_unittests, angle_end2end_tests, WebGL conformance tests

Change-Id: Iec727821d8137db56b440ddbe007879b1b55f61f
Reviewed-on: https://chromium-review.googlesource.com/707195
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>