Add forward propagation to SLP "any" permutes

Add forward propagation to SLP "any" permutes

This adds a forward propagation phase to the permute optimization
machinery which allows us to handle "any" permute for all kinds of
nodes.  To match previous behavior cost-wise we still do not allow
non-external/constant nodes to be duplicated for multiple permutes
and this is ensured during propagation itself.

2021-06-29  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_optimize_slp): Forward propagate
	to "any" permute nodes and relax "any" permute proapgation
	during iterative backward propagation.