Add x86 addsub SLP pattern

This addds SLP pattern recognition for the SSE3/AVX [v]addsubp{ds} v0, v1
instructions which compute { v0[0] - v1[0], v0[1], + v1[1], ... }
thus subtract, add alternating on lanes, starting with subtract.

It adds a corresponding optab and direct internal function,
vec_addsub$a3 and renames the existing i386 backend patterns to
the new canonical name.

The SLP pattern matches the exact alternating lane sequence rather
than trying to be clever and anticipating incoming permutes - we
could permute the two input vectors to the needed lane alternation,
do the addsub and then permute the result vector back but that's
only profitable in case the two input or the output permute will
vanish - something Tamars refactoring of SLP pattern recog should
make possible.

2021-06-17  Richard Biener  <rguenther@suse.de>

	* config/i386/sse.md (avx_addsubv4df3): Rename to
	vec_addsubv4df3.
	(avx_addsubv8sf3): Rename to vec_addsubv8sf3.
	(sse3_addsubv2df3): Rename to vec_addsubv2df3.
	(sse3_addsubv4sf3): Rename to vec_addsubv4sf3.
	* config/i386/i386-builtin.def: Adjust.
	* internal-fn.def (VEC_ADDSUB): New internal optab fn.
	* optabs.def (vec_addsub_optab): New optab.
	* tree-vect-slp-patterns.c (class addsub_pattern): New.
	(slp_patterns): Add addsub_pattern.
	* tree-vect-slp.c (vect_optimize_slp): Disable propagation
	across CFN_VEC_ADDSUB.
	* tree-vectorizer.h (vect_pattern::vect_pattern): Make
	m_ops optional.
	* doc/md.texi (vec_addsub<mode>3): Document.

	* gcc.target/i386/vect-addsubv2df.c: New testcase.
	* gcc.target/i386/vect-addsubv4sf.c: Likewise.
	* gcc.target/i386/vect-addsubv4df.c: Likewise.
	* gcc.target/i386/vect-addsubv8sf.c: Likewise.
	* gcc.target/i386/vect-addsub-2.c: Likewise.
	* gcc.target/i386/vect-addsub-3.c: Likewise.