aarch64: Reimplement vmovn_high_* intrinsics using builtins

aarch64: Reimplement vmovn_high_* intrinsics using builtins

The vmovn_high* intrinsics are supposed to map to XTN2 instructions that
narrow their source vector and instert it into the top half of the destination vector.
This patch reimplements them away from inline assembly to an RTL builtin
that performs a vec_concat with a truncate.

gcc/
	* config/aarch64/aarch64-simd.md (aarch64_xtn2<mode>_le):
	Define.
	(aarch64_xtn2<mode>_be): Likewise.
	(aarch64_xtn2<mode>): Likewise.
	* config/aarch64/aarch64-simd-builtins.def (xtn2): Define
	builtins.
	* config/aarch64/arm_neon.h (vmovn_high_s16): Reimplement using
	builtins.
	(vmovn_high_s32): Likewise.
	(vmovn_high_s64): Likewise.
	(vmovn_high_u16): Likewise.
	(vmovn_high_u32): Likewise.
	(vmovn_high_u64): Likewise.

gcc/testsuite/
	* gcc.target/aarch64/narrow_high-intrinsics.c: Adjust
	scan-assembler-times for xtn2.