Add FMADDSUB and FMSUBADD SLP vectorization patterns and optabs

Add FMADDSUB and FMSUBADD SLP vectorization patterns and optabs

This adds named expanders for vec_fmaddsub<mode>4 and
vec_fmsubadd<mode>4 which map to x86 vfmaddsubXXXp{ds} and
vfmsubaddXXXp{ds} instructions.  This complements the previous
addition of ADDSUB support.

x86 lacks SUBADD and the negate variants of FMA with mixed
plus minus so I did not add optabs or patterns for those but
it would not be difficult if there's a target that has them.

2021-07-05  Richard Biener  <rguenther@suse.de>

	* doc/md.texi (vec_fmaddsub<mode>4): Document.
	(vec_fmsubadd<mode>4): Likewise.
	* optabs.def (vec_fmaddsub$a4): Add.
	(vec_fmsubadd$a4): Likewise.
	* internal-fn.def (IFN_VEC_FMADDSUB): Add.
	(IFN_VEC_FMSUBADD): Likewise.
	* tree-vect-slp-patterns.c (addsub_pattern::recognize):
	Refactor to handle IFN_VEC_FMADDSUB and IFN_VEC_FMSUBADD.
	(addsub_pattern::build): Likewise.
	* tree-vect-slp.c (vect_optimize_slp): CFN_VEC_FMADDSUB
	and CFN_VEC_FMSUBADD are not transparent for permutes.
	* config/i386/sse.md (vec_fmaddsub<mode>4): New expander.
	(vec_fmsubadd<mode>4): Likewise.

	* gcc.target/i386/vect-fmaddsubXXXpd.c: New testcase.
	* gcc.target/i386/vect-fmaddsubXXXps.c: Likewise.
	* gcc.target/i386/vect-fmsubaddXXXpd.c: Likewise.
	* gcc.target/i386/vect-fmsubaddXXXps.c: Likewise.