glsl: Small optimization for constant conditionals

Once the relevant branch has been identified do not iterate over the
instructions in the branch, do a linked list insertion instead to avoid the
loop.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/opt_if_simplification.cpp

@@ -90,15 +90,9 @@ ir_if_simplification_visitor::visit_leave(ir_if *ir)
        * that matters out.
        */
       if (condition_constant->value.b[0]) {
-	 foreach_list_safe(n, &ir->then_instructions) {
-	    ir_instruction *then_ir = (ir_instruction *) n;
-	    ir->insert_before(then_ir);
-	 }
+         ir->insert_before(&ir->then_instructions);
       } else {
-	 foreach_list_safe(n, &ir->else_instructions) {
-	    ir_instruction *else_ir = (ir_instruction *) n;
-	    ir->insert_before(else_ir);
-	 }
+         ir->insert_before(&ir->else_instructions);
       }
       ir->remove();
       this->made_progress = true;