tree-optimization/117567 - make SLP reassoc resilent against NULL lanes

tree-optimization/117567 - make SLP reassoc resilent against NULL lanes

The following tries to make the SLP chain association code resilent
against not present lanes (the other option would have been to disable
it in this case).  Not present lanes can now more widely appear as
part of mask load SLP discovery when there is gaps involved.  Requiring
a present first lane shouldn't be a restriction since in unpermuted
state all DR groups have their first lane not a gap.

	PR tree-optimization/117567
	* tree-vect-slp.cc (vect_build_slp_tree_2): Handle not present
	lanes when doing re-association.

	* gcc.dg/torture/pr117567.c: New testcase.