glsl: Skip the rest of loop unrolling if no loops were found.

Shaves 1.6% (+/- 1.0%) off of ff_fragment_shader glean texCombine time
(n=5).

src/glsl/loop_analysis.h

@@ -214,6 +214,8 @@ public:
 
    loop_variable_state *insert(ir_loop *ir);
 
+   bool loop_found;
+
 private:
    loop_state();