aarch64: Reimplement vrshrn* intrinsics using builtins

aarch64: Reimplement vrshrn* intrinsics using builtins

This patch moves the vrshrn* intrinsics to builtins away from inline
asm.

It's a bit of code, but it's very similar to the recent vsrhn*
reimplementation except that we use an unspec rather than standard RTL
codes for the functionality.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def (rshrn, rshrn2):
	Define builtins.
	* config/aarch64/aarch64-simd.md (aarch64_rshrn<mode>_insn_le):
	Define.
	(aarch64_rshrn<mode>_insn_be): Likewise.
	(aarch64_rshrn<mode>): Likewise.
	(aarch64_rshrn2<mode>_insn_le): Likewise.
	(aarch64_rshrn2<mode>_insn_be): Likewise.
	(aarch64_rshrn2<mode>): Likewise.
	* config/aarch64/aarch64.md (unspec): Add UNSPEC_RSHRN.
	* config/aarch64/arm_neon.h (vrshrn_high_n_s16): Reimplement
	using builtin.
	(vrshrn_high_n_s32): Likewise.
	(vrshrn_high_n_s64): Likewise.
	(vrshrn_high_n_u16): Likewise.
	(vrshrn_high_n_u32): Likewise.
	(vrshrn_high_n_u64): Likewise.
	(vrshrn_n_s16): Likewise.
	(vrshrn_n_s32): Likewise.
	(vrshrn_n_s64): Likewise.
	(vrshrn_n_u16): Likewise.
	(vrshrn_n_u32): Likewise.
	(vrshrn_n_u64): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/narrow_high-intrinsics.c: Adjust rshrn2
	assembly scan.