Make vect_get_mask_type_for_stmt take a group size

Make vect_get_mask_type_for_stmt take a group size

This patch makes vect_get_mask_type_for_stmt and
get_mask_type_for_scalar_type take a group size instead of
the SLP node, so that later patches can call it before an
SLP node has been built.

2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (get_mask_type_for_scalar_type): Replace
	the slp_tree parameter with a group size parameter.
	(vect_get_mask_type_for_stmt): Likewise.
	* tree-vect-stmts.c (get_mask_type_for_scalar_type): Likewise.
	(vect_get_mask_type_for_stmt): Likewise.
	* tree-vect-slp.c (vect_slp_analyze_node_operations_1): Update
	call accordingly.

From-SVN: r278849