Consistently pass the vector type for scalar SLP cost compute

Consistently pass the vector type for scalar SLP cost compute

This avoids randomly (based on whether the stmt is
SLP_TREE_REPRESENTATIVE and not a pattern stmt) passing a vector
type or NULL to the add_stmt_cost hook for scalar code cost
compute.  For example the x86 backend uses only the vector type to
decide on the scalar computation mode which makes costing off.

So the following explicitely passes the vector type and uses
SLP_TREE_VECTYPE for this purpose.

2020-10-29  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_bb_slp_scalar_cost): Pass
	SLP_TREE_VECTYPE to record_stmt_cost.