Adjust BB vectorization function splitting

Adjust BB vectorization function splitting

This adjusts the condition when to split at control altering stmts,
only when there's a definition.  It also removes the only use
of --param slp-max-insns-in-bb which a previous change left doing
nothing (but repeatedly print a message for each successive
instruction...).

2020-10-27  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_slp_bbs): Remove no-op
	slp-max-insns-in-bb check.
	(vect_slp_function): Dump when splitting the function.
	Adjust the split condition for control altering stmts.
	* params.opt (-param=slp-max-insns-in-bb): Remove.
	* doc/invoke.texi (-param=slp-max-insns-in-bb): Likewise.