xtensa: Make use of std::swap where appropriate

No functional changes.

gcc/ChangeLog:

	* config/xtensa/xtensa.cc
	(gen_int_relational, gen_float_relational): Replace tempvar-based
	value-swapping codes with std::swap.
	* config/xtensa/xtensa.md (movdi_internal, movdf_internal):
	Ditto.