fix useless unsharing of SLP tree

fix useless unsharing of SLP tree

This avoids unsharing the SLP tree when optimizing load permutations
for reductions but there is no actual permute taking place.

2020-09-09  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_attempt_slp_rearrange_stmts): Do
	nothing when the permutation doesn't permute.