Treat return statements in loops as loop discontinuities.

This is so that texture2D is converted to texture2DLod in loops with return statements. HLSL does not seem to allow gradient operations in loops with return statements. It considers them to be breaks.
Review URL: https://codereview.appspot.com/7131057

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1722 736b8ea6-26fd-11df-bfd4-992fa37f6226