glsl: Fix ir_last_opcode value.

Now that ir_quadop_vector exists, ir_last_binop and ir_last_opcode are
no longer the same.  Only one place currently uses this enumeration, and
already handles ir_quadop_vector correctly.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Olivier Galibert <galibert@pobox.com>

src/glsl/ir.h

@@ -1027,7 +1027,7 @@ enum ir_expression_operation {
    /**
     * A sentinel marking the last of all operations.
     */
-   ir_last_opcode = ir_last_binop
+   ir_last_opcode = ir_quadop_vector
 };
 
 class ir_expression : public ir_rvalue {