aarch64: Use type-qualified builtins for [R]SUBHN[2] Neon intrinsics

aarch64: Use type-qualified builtins for [R]SUBHN[2] Neon intrinsics

Declare unsigned type-qualified builtins and use them to implement
(rounding) halving-narrowing-subtract Neon intrinsics. This removes
the need for many casts in arm_neon.h.

gcc/ChangeLog:

2021-11-09  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Declare unsigned
	builtins for [r]subhn[2].
	* config/aarch64/arm_neon.h (vsubhn_s16): Remove unnecessary
	cast.
	(vsubhn_s32): Likewise.
	(vsubhn_s64): Likewise.
	(vsubhn_u16): Use type-qualified builtin and remove casts.
	(vsubhn_u32): Likewise.
	(vsubhn_u64): Likewise.
	(vrsubhn_s16): Remove unnecessary cast.
	(vrsubhn_s32): Likewise.
	(vrsubhn_s64): Likewise.
	(vrsubhn_u16): Use type-qualified builtin and remove casts.
	(vrsubhn_u32): Likewise.
	(vrsubhn_u64): Likewise.
	(vrsubhn_high_s16): Remove unnecessary cast.
	(vrsubhn_high_s32): Likewise.
	(vrsubhn_high_s64): Likewise.
	(vrsubhn_high_u16): Use type-qualified builtin and remove
	casts.
	(vrsubhn_high_u32): Likewise.
	(vrsubhn_high_u64): Likewise.
	(vsubhn_high_s16): Remove unnecessary cast.
	(vsubhn_high_s32): Likewise.
	(vsubhn_high_s64): Likewise.
	(vsubhn_high_u16): Use type-qualified builtin and remove
	casts.
	(vsubhn_high_u32): Likewise.
	(vsubhn_high_u64): Likewise.