aarch64: Factorise widening add/sub high-half expanders with iterators

I noticed these define_expand are almost identical modulo some string substitutions.
This patch compresses them together with a couple of code iterators.
No functional change intended.
Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (aarch64_saddw2<mode>): Delete.
	(aarch64_uaddw2<mode>): Delete.
	(aarch64_ssubw2<mode>): Delete.
	(aarch64_usubw2<mode>): Delete.
	(aarch64_<ANY_EXTEND:su><ADDSUB:optab>w2<mode>): New define_expand.