aarch64: reimplement vqmovn_high* intrinsics using builtins

aarch64: reimplement vqmovn_high* intrinsics using builtins

This patch reimplements the saturating-truncate-and-insert-into-high
intrinsics using the appropriate RTL codes and builtins.

gcc/
	* config/aarch64/aarch64-simd.md (aarch64_<su>qxtn2<mode>_le):
	Define.
	(aarch64_<su>qxtn2<mode>_be): Likewise.
	(aarch64_<su>qxtn2<mode>): Likewise.
	* config/aarch64/aarch64-simd-builtins.def (sqxtn2, uqxtn2):
	Define builtins.
	* config/aarch64/iterators.md (SAT_TRUNC): Define code_iterator.
	(su): Handle ss_truncate and us_truncate.
	* config/aarch64/arm_neon.h (vqmovn_high_s16): Reimplement using
	builtin.
	(vqmovn_high_s32): Likewise.
	(vqmovn_high_s64): Likewise.
	(vqmovn_high_u16): Likewise.
	(vqmovn_high_u32): Likewise.
	(vqmovn_high_u64): Likewise.

gcc/testsuite/
	* gcc.target/aarch64/narrow_high-intrinsics.c: Update uqxtn2 and
	sqxtn2 scan-assembler-times.