tree-optimization/116575 - SLP masked load-lanes discovery

tree-optimization/116575 - SLP masked load-lanes discovery

The following implements masked load-lane discovery for SLP.  The
challenge here is that a masked load has a full-width mask with
group-size number of elements when this becomes a masked load-lanes
instruction one mask element gates all group members.  We already
have some discovery hints in place, namely STMT_VINFO_SLP_VECT_ONLY
to guard non-uniform masks, but we need to choose a way for SLP
discovery to handle possible masked load-lanes SLP trees.

I have this time chosen to handle load-lanes discovery where we
have performed permute optimization already and conveniently got
the graph with predecessor edges built.  This is because unlike
non-masked loads masked loads with a load_permutation are never
produced by SLP discovery (because load permutation handling doesn't
handle un-permuting the mask) and thus the load-permutation lowering
which handles non-masked load-lanes discovery doesn't trigger.

With this SLP discovery for a possible masked load-lanes, thus
a masked load with uniform mask, produces a splat of a single-lane
sub-graph as the mask SLP operand.  This is a representation that
shouldn't pessimize the mask load case and allows the masked load-lanes
transform to simply elide this splat.

This fixes the aarch64-sve.exp mask_struct_load*.c testcases with
--param vect-force-slp=1

	PR tree-optimization/116575
	* tree-vect-slp.cc (vect_get_and_check_slp_defs): Handle
	gaps, aka NULL scalar stmt.
	(vect_build_slp_tree_2): Allow gaps in the middle of a
	grouped mask load.  When the mask of a grouped mask load
	is uniform do single-lane discovery for the mask and
	insert a splat VEC_PERM_EXPR node.
	(vect_optimize_slp_pass::decide_masked_load_lanes): New
	function.
	(vect_optimize_slp_pass::run): Call it.