ForLoopUnroll should only mark loops that fit the limitations in ESSL
1.00 Appendix A.

BUG=angleproject:1253
TEST=angle_unittests, WebGL conformance tests

Change-Id: I00b0a7d29cd42efea9611d020aa1f873ac04773f
Reviewed-on: https://chromium-review.googlesource.com/317551Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>