aarch64: Improve swp generation

Allow zero as an input; fix constraints; avoid unnecessary split.

	* config/aarch64/aarch64.c (aarch64_emit_atomic_swap): Remove.
	(aarch64_gen_atomic_ldop): Don't call it.
	* config/aarch64/atomics.md (atomic_exchange<ALLI>):
	Use aarch64_reg_or_zero.
	(aarch64_atomic_exchange<ALLI>): Likewise.
	(aarch64_atomic_exchange<ALLI>_lse): Remove split; remove & from
	operand 0; use aarch64_reg_or_zero for input; merge ...
	(@aarch64_atomic_swp<ALLI>): ... this and remove.

From-SVN: r265659