tree-optimization/101615 - SLP permute opt of existing vectors

tree-optimization/101615 - SLP permute opt of existing vectors

This fixes one issue discovered when analyzing PR101615, namely
we happily push permutes to pre-existing vectors but end up
not actually permuting them.  In fact we don't want to, so force
materialization on the external.

It doesn't fix the original testcase though.

2021-07-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101615
	* tree-vect-slp.c (vect_optimize_slp): Pre-existing vector
	external nodes cannot be permuted so make them perm_out 0.

	* gcc.dg/vect/bb-slp-pr101615-1.c: New testcase.