tree-optimization/93868 copy SLP tree before re-arranging stmts

tree-optimization/93868 copy SLP tree before re-arranging stmts

This avoids altering possibly shared SLP subtrees when attempting
to get rid of permutations in SLP reductions by copying the SLP
subtree before re-arranging stmts in it.

2020-02-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93868
	* tree-vect-slp.c (slp_copy_subtree): New function.
	(vect_attempt_slp_rearrange_stmts): Copy the SLP tree before
	re-arranging stmts in it.

	* gcc.dg/torture/pr93868.c: New testcase.