tree-optimization/116083 - improve behavior when SLP discovery limit is reached

tree-optimization/116083 - improve behavior when SLP discovery limit is reached

The following avoids some useless work when the SLP discovery limit
is reached, for example allocating a node to cache the failure
and starting discovery on split store groups when analyzing BBs.

It does not address the issue in the PR which is a gratious budget
for discovery when the store group size approaches the number of
overall statements.

	PR tree-optimization/116083
	* tree-vect-slp.cc (vect_build_slp_tree): Do not allocate
	a discovery fail node when we reached the discovery limit.
	(vect_build_slp_instance): Terminate early when the
	discovery limit is reached.