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

Helps one program in shader-db:

instructions in affected programs:     96 -> 92 (-4.17%)

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

src/glsl/opt_algebraic.cpp

@@ -570,6 +570,8 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
          return ir->operands[0];
       } else if (is_vec_zero(op_const[0])) {
          return mul(ir->operands[1], ir->operands[2]);
+      } else if (is_vec_zero(op_const[1])) {
+         return add(ir->operands[0], neg(mul(ir->operands[0], ir->operands[2])));
       }
       break;