tree-optimization/95866 - avoid using scalar ops for vectorized shift

tree-optimization/95866 - avoid using scalar ops for vectorized shift

This avoids using the original scalar SSA operand when vectorizing
a shift with a vectorized shift operand where we know all vector
components have the same value and thus we can use a vector by
scalar shift.  Using the scalar SSA operand causes a possibly
long chain of scalar computation to be retained so it's better
to simply extract lane zero from the available vectorized shift
operand.

2020-06-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95866
	* tree-vect-stmts.c (vectorizable_shift): Reject incompatible
	vectorized shift operands.  For scalar shifts use lane zero
	of a vectorized shift operand.

	* gcc.dg/vect/bb-slp-pr95866.c: New testcase.