aarch64: Relax aarch64_<sur><addsub>hn2<mode> RTL pattern

Implement v[r]addhn2 and v[r]subhn2 Neon intrinsic RTL patterns using
a vec_concat of a register_operand and an ADDSUBHN unspec - instead
of just an ADDSUBHN2 unspec. This more relaxed pattern allows for
more aggressive combinations and ultimately better code generation.

This patch also removes the now redundant [R]ADDHN2 and [R]SUBHN2
unspecs and their iterator.

gcc/ChangeLog:

2021-03-03  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd.md (aarch64_<sur><addsub>hn2<mode>):
	Implement as an expand emitting a big/little endian
	instruction pattern.
	(aarch64_<sur><addsub>hn2<mode>_insn_le): Define.
	(aarch64_<sur><addsub>hn2<mode>_insn_be): Define.
	* config/aarch64/iterators.md: Remove UNSPEC_[R]ADDHN2 and
	UNSPEC_[R]SUBHN2 unspecs and ADDSUBHN2 iterator.