[GCC][AARCH64] Canonicalize aarch64 widening simd plus insns

[GCC][AARCH64] Canonicalize aarch64 widening simd plus insns

Committed on behalf of matthew.malcomson@arm.com

2018-07-24  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/aarch64/aarch64-simd.md
	(aarch64_<ANY_EXTEND:su><ADDSUB:optab>w<mode>): Split into...
	(aarch64_<ANY_EXTEND:su>subw<mode>): ... This...
	(aarch64_<ANY_EXTEND:su>addw<mode>): ... And this.
	(aarch64_<ANY_EXTEND:su><ADDSUB:optab>w<mode>_internal): Split into...
	(aarch64_<ANY_EXTEND:su>subw<mode>_internal): ... This...
	(aarch64_<ANY_EXTEND:su>addw<mode>_internal): ... And this.
	(aarch64_<ANY_EXTEND:su><ADDSUB:optab>w2<mode>_internal): Split into...
	(aarch64_<ANY_EXTEND:su>subw2<mode>_internal): ... This...
	(aarch64_<ANY_EXTEND:su>addw2<mode>_internal): ... And this.

	* gcc.target/aarch64/vect-su-add-sub.c: New.

From-SVN: r262949