glsl/loops: Remove unnecessary list walk from loop_control_visitor.

When loop_control_visitor::visit_leave(ir_loop *) is analyzing a loop
terminator that acts on a certain ir_variable, it doesn't need to walk
the list of induction variables to find the loop_variable entry
corresponding to the variable.  It can just look it up in the
loop_variable_state hashtable and verify that the loop_variable entry
represents an induction variable.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/loop_analysis.h

@@ -189,6 +189,15 @@ public:
    ir_rvalue *increment;
 
 
+   inline bool is_induction_var() const
+   {
+      /* Induction variables always have a non-null increment, and vice
+       * versa.
+       */
+      return this->increment != NULL;
+   }
+
+
    inline bool is_loop_constant() const
    {
       const bool is_const = (this->num_assignments == 0)