aarch64: Improve representation of vpaddd intrinsics

The aarch64_addpdi pattern is redundant as the reduc_plus_scal_<mode> pattern can already generate
the required form of the ADDP instruction, and is mostly folded to GIMPLE early on so can benefit from more optimisations.
Though it turns out that we were missing the folding for the unsigned variants.
This patch adds that and wires up the vpaddd_u64 and vpaddd_s64 intrinsics through the above pattern instead
so that we can remove a redundant pattern and get more optimisation earlier.

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

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.cc (aarch64_general_gimple_fold_builtin):
	Handle unsigned reduc_plus_scal_ builtins.
	* config/aarch64/aarch64-simd-builtins.def (addp): Delete DImode instances.
	* config/aarch64/aarch64-simd.md (aarch64_addpdi): Delete.
	* config/aarch64/arm_neon.h (vpaddd_s64): Reimplement with
	__builtin_aarch64_reduc_plus_scal_v2di.
	(vpaddd_u64): Reimplement with __builtin_aarch64_reduc_plus_scal_v2di_uu.