tree-optimization/115602 - SLP CSE results in cycles

tree-optimization/115602 - SLP CSE results in cycles

The following prevents SLP CSE to create new cycles which happened
because of a 1:1 permute node being present where its child was then
CSEd to the permute node.  Fixed by making a node only available to
CSE to after recursing.

	PR tree-optimization/115602
	* tree-vect-slp.cc (vect_cse_slp_nodes): Delay populating the
	bst-map to avoid cycles.

	* gcc.dg/vect/pr115602.c: New testcase.