tree-optimization/119640 - ICE with vectorized shift placement

tree-optimization/119640 - ICE with vectorized shift placement

When the whole shift is invariant but the shift amount needs
to be converted and a vector shift used we can mess up placement
of vector stmts because we do not make SLP scheduling aware of
the need to insert code for it.  The following mitigates this
by more conservative placement of such code in vectorizable_shift.

	PR tree-optimization/119640
	* tree-vect-stmts.cc (vectorizable_shift): Always insert code
	for one of our SLP operands before the code for the vector
	shift itself.

	* gcc.dg/vect/pr119640.c: New testcase.