glsl: Optimize lrp(0, y, a) into y * a.

Helps two programs in shader-db:

instructions in affected programs:     254 -> 234 (-7.87%)

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

src/glsl/opt_algebraic.cpp

@@ -568,6 +568,8 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
          return ir->operands[1];
       } else if (ir->operands[0]->equals(ir->operands[1])) {
          return ir->operands[0];
+      } else if (is_vec_zero(op_const[0])) {
+         return mul(ir->operands[1], ir->operands[2]);
       }
       break;