[2/2] aarch64: Reimplement (R){ADD,SUB}HN2 patterns with standard RTL codes

Similar to the previous patch, this one converts the high-half versions of the patterns.
With this patch we can remove the UNSPEC_* codes involved entirely.

Bootstrapped and tested on aarch64-none-linux-gnu. Also tested on aarch64_be-none-elf.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (aarch64_<sur><addsub>hn2<mode>_insn_le):
	Rename and reimplement with RTL codes to...
	(aarch64_<optab>hn2<mode>_insn_le): .. This.
	(aarch64_r<optab>hn2<mode>_insn_le): New pattern.
	(aarch64_<sur><addsub>hn2<mode>_insn_be): Rename and reimplement with RTL
	codes to...
	(aarch64_<optab>hn2<mode>_insn_be): ... This.
	(aarch64_r<optab>hn2<mode>_insn_be): New pattern.
	(aarch64_<sur><addsub>hn2<mode>): Rename and adjust expander to...
	(aarch64_<optab>hn2<mode>): ... This.
	(aarch64_r<optab>hn2<mode>): New expander.
	* config/aarch64/iterators.md (UNSPEC_ADDHN, UNSPEC_RADDHN,
	UNSPEC_SUBHN, UNSPEC_RSUBHN): Delete unspecs.
	(ADDSUBHN): Delete.
	(sur): Remove handling of the above.
	(addsub): Likewise.