aarch64: Reimplement vqmovun_high* intrinsics using builtins

aarch64: Reimplement vqmovun_high* intrinsics using builtins

Another transition from inline asm to builtin.
Only 3 intrinsics converted this time but they use the "+w" constraint in their inline asm
so are more likely to generate redundant moves so benefit more from reimplementation.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def (sqxtun2): Define builtin.
	* config/aarch64/aarch64-simd.md (aarch64_sqxtun2<mode>_le): Define.
	(aarch64_sqxtun2<mode>_be): Likewise.
	(aarch64_sqxtun2<mode>): Likewise.
	* config/aarch64/arm_neon.h (vqmovun_high_s16): Reimplement using builtin.
	(vqmovun_high_s32): Likewise.
	(vqmovun_high_s64): Likewise.
	* config/aarch64/iterators.md (UNSPEC_SQXTUN2): Define.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/narrow_high-intrinsics.c: Adjust sqxtun2 scan.