tree-optimization/124743 - SLP scheduling of invariant internal ops

tree-optimization/124743 - SLP scheduling of invariant internal ops

The following makes us avoid scheduling invariant internal operations
to random points in the CFG when doing loop vectorization since
in that case the stmt UIDs we use for dominance checks are not
initialized outside of loop bodies.  Instead schedule such
operations by appending to the loop preheader which is where
invariants generally end up for loop vectorization.

	PR tree-optimization/124743
	* tree-vect-slp.cc (vect_schedule_slp_node): Schedule
	loop invariant operations in the loop preheader.

	* gcc.dg/vect/vect-pr124743.c: New testcase.