AArch64: Correct dot-product auto-vect optab RTL

AArch64: Correct dot-product auto-vect optab RTL

The current RTL for the vectorizer patterns for dot-product are incorrect.
Operand3 isn't an output parameter so we can't write to it.

This fixes this issue and reduces the number of RTL.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def (udot, sdot): Rename to...
	(sdot_prod, udot_prod): ...These.
	* config/aarch64/aarch64-simd.md (<sur>dot_prod<vsi2qi>): Remove.
	(aarch64_<sur>dot<vsi2qi>): Rename to...
	(<sur>dot_prod<vsi2qi>): ...This.
	* config/aarch64/arm_neon.h (vdot_u32, vdotq_u32, vdot_s32, vdotq_s32):
	Update builtins.