tree-optimization/96022 - fix ICE with vectorized shift

tree-optimization/96022 - fix ICE with vectorized shift

This fixes lane extraction for internal def vectorized shifts
with an effective scalar shift operand by always using lane zero
of the first vector stmt.

It also fixes a SLP build issue noticed on the testcase where
we end up building unary vector ops with the only operand built
form scalars which isn't profitable by itself.  The exception
is for stores.

2020-07-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96022
	* tree-vect-stmts.c (vectorizable_shift): Only use the
	first vector stmt when extracting the scalar shift amount.
	* tree-vect-slp.c (vect_build_slp_tree_2): Also build unary
	nodes with all-scalar children from scalars but not stores.
	(vect_analyze_slp_instance): Mark the node not failed.

	* g++.dg/vect/pr96022.cc: New testcase.