aarch64: Add =r,m and =m,r alternatives to 64-bit vector move patterns

We can use the X registers to load and store 64-bit vector modes, we just need to add the alternatives
to the mov patterns. This straightforward patch does that and for the pair variants too.
For the testcase in the code we now generate the optimal assembly without any superfluous
GP<->SIMD moves.

Bootstrapped and tested on aarch64-none-linux-gnu and aarch64_be-none-elf.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (*aarch64_simd_mov<VDMOV:mode>):
	Add =r,m and =r,m alternatives.
	(load_pair<DREG:mode><DREG2:mode>): Likewise.
	(vec_store_pair<DREG:mode><DREG2:mode>): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/xreg-vec-modes_1.c: New test.