tree-optimization/95356 - more vectorizable_shift massaging

The previous fix clashed with the rewrite to emit SLP invariants
during the SLP walk.  Thus the following adjusts the SLP tree
hacking vectorizable_shift does appropriately.

Still resisting the attempt of a rewrite of vectorizable_shift ...

2020-05-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95356
	* tree-vect-stmts.c (vectorizable_shift): Do in-place SLP
	node hacking during analysis.