[AArch64] Add support for 64-bit vector-mode ldp/stp

	* config/aarch64/aarch64.c (aarch64_mode_valid_for_sched_fusion_p):
	New function.
	(fusion_load_store): Use it.
	* config/aarch64/aarch64-ldpstp.md: Add new peephole2s for
	ldp and stp in VD modes.
	* config/aarch64/aarch64-simd.md (load_pair<mode>, VD): New pattern.
	(store_pair<mode>, VD): Likewise.

	* gcc.target/aarch64/stp_vec_64_1.c: New test.
	* gcc.target/aarch64/ldp_vec_64_1.c: Likewise.

From-SVN: r229094