rs6000: Clean up the cstore code a bit

rs6000: Clean up the cstore code a bit

"register_operand" was a bit confusing.  Also some other minor cleanups.

	* (cstore<mode>4_unsigned): Use gpc_reg_operand instead of
	register_operand.  Remove empty constraints.  Use std::swap.
	(cstore_si_as_di, cstore<mode>4_signed_imm,
	cstore<mode>4_unsigned_imm, cstore<mode>4 for GPR): Use
	gpc_reg_operand instead of register_operand.
	(cstore<mode>4 for FP): Use gpc_reg_operand instead of
	register_operand.  Remove empty constraints.

From-SVN: r231287