tree-optimization/98544 - more permute optimization fixes

tree-optimization/98544 - more permute optimization fixes

Permute nodes are not transparent to the permute of their children.
Instead we have to materialize child permutes always and in future
may treat permute nodes as the source of arbitrary permutes as
we can permute the lane permutation vector at will (as the target
supports in the end).

2021-01-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98544
	* tree-vect-slp.c (vect_optimize_slp): Always materialize
	permutes at a permute node.

	* gcc.dg/vect/bb-slp-pr98544.c: New testcase.