tree-optimization/102128 - rework if-converted BB vect heuristic

tree-optimization/102128 - rework if-converted BB vect heuristic

This reworks the previous attempt to avoid leaving around if-converted
scalar code in BB vectorized loop bodies to keep costing independent
subgraphs which should address the observed regression with 519.lbm_r.

For this to work we now first cost all subgraphs and only after
doing that proceed to emit vectorized code.

2021-08-30  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102128
	* tree-vect-slp.c (vect_bb_vectorization_profitable_p):
	Move scanning for if-converted scalar code to the caller
	and instead delay clearing the visited flag for profitable
	subgraphs.
	(vect_slp_region): Cost all subgraphs before scheduling.
	For if-converted BB vectorization scan for scalar COND_EXPRs
	and do not vectorize if any found and the cost model is
	very-cheap.