tree-optimization/112404 - two issues with SLP of .MASK_LOAD

tree-optimization/112404 - two issues with SLP of .MASK_LOAD

The following fixes an oversight in vect_check_scalar_mask when
the mask is external or constant.  When doing BB vectorization
we need to provide a group_size, best via an overload accepting
the SLP node as argument.

When fixed we then run into the issue that we have not analyzed
alignment of the .MASK_LOADs because they were not identified
as loads by vect_gather_slp_loads.  Fixed by reworking the
detection.

	PR tree-optimization/112404
	* tree-vectorizer.h (get_mask_type_for_scalar_type): Declare
	overload with SLP node argument.
	* tree-vect-stmts.cc (get_mask_type_for_scalar_type): Implement it.
	(vect_check_scalar_mask): Use it.
	* tree-vect-slp.cc (vect_gather_slp_loads): Properly identify
	loads also for nodes with children, like .MASK_LOAD.
	* tree-vect-loop.cc (vect_analyze_loop_2): Look at the
	representative for load nodes and check whether it is a grouped
	access before looking for load-lanes support.

	* gfortran.dg/pr112404.f90: New testcase.