tree-optimization/102176 - locally compute participating SLP stmts

This performs local re-computation of participating scalar stmts
in BB vectorization subgraphs to allow precise computation of
liveness of scalar stmts after vectorization and thus precise
costing.  This treats all extern defs as live but continues
to optimistically handle scalar defs that we think we can handle
by lane-extraction even though that can still fail late during
code-generation.

2021-09-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102176
	* tree-vect-slp.c (vect_slp_gather_vectorized_scalar_stmts):
	New function.
	(vect_bb_slp_scalar_cost): Use the computed set of
	vectorized scalar stmts instead of relying on the out-of-date
	and not accurate PURE_SLP_STMT.
	(vect_bb_vectorization_profitable_p): Compute the set
	of vectorized scalar stmts.

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