Avoid representing SLP mask by scalar op

Avoid representing SLP mask by scalar op

The following removes the scalar mask output from vect_check_scalar_mask
and deals with the fallout, eliminating uses of it.  That's mostly
replacing checks on 'mask' by checks on 'mask_node' but also realizing
PR121349 and fixing that up a bit in check_load_store_for_partial_vectors.

	PR tree-optimization/121349
	* tree-vect-stmts.cc (check_load_store_for_partial_vectors):
	Get full SLP mask, reduce to uniform scalar_mask for further
	processing if possible.
	(vect_check_scalar_mask): Remove scalar mask output, remove
	code conditional on slp_mask.
	(vectorizable_call): Adjust.
	(check_scan_store): Get and check SLP mask.
	(vectorizable_store): Eliminate scalar mask variable.
	(vectorizable_load): Likewise.