aarch64: Add V1DI mode

aarch64: Add V1DI mode

We already have a V1DF mode, so this makes the vector modes more consistent.

Additionally, this allows us to recognise uint64x1_t and int64x1_t types given
only the mode and type qualifiers (e.g. in aarch64_lookup_simd_builtin_type).

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.cc
	(v1di_UP): Add V1DI mode to _UP macros.
	* config/aarch64/aarch64-modes.def (VECTOR_MODE): Add V1DI mode.
	* config/aarch64/aarch64-simd-builtin-types.def: Use V1DI mode.
	* config/aarch64/aarch64-simd.md
	(vec_extractv2dfv1df): Replace with...
	(vec_extract<mode><V1half>): ...this.
	* config/aarch64/aarch64.cc
	(aarch64_classify_vector_mode): Add V1DI mode.
	* config/aarch64/iterators.md
	(VQ_2E, V1HALF, V1half): New.
	(nunits): Add V1DI mode.