When possibly expanding a hash-map avoid keeping a reference to an
entry.

2020-02-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93953
	* tree-vect-slp.c (slp_copy_subtree): Avoid keeping a reference
	to the hash-map entry.

	* gcc.dg/pr93953.c: New testcase.