tree-optimization/119960 - add validity checking to SLP scheduling

tree-optimization/119960 - add validity checking to SLP scheduling

The following adds checks that when we search for a vector stmt
insert location we arrive at one where all required operand defs
are dominating the insert location.  At the moment any such
failure only blows up during SSA verification.

There's the long-standing issue that we do not verify there
exists a valid schedule of the SLP graph from BB vectorization
into the existing CFG.  We do not have the ability to insert
vector stmts on the dominance frontier "end", nor to insert
LC PHIs that would be eventually required.

This should be done all differently, computing the schedule
during analysis and failing if we can't schedule.

	PR tree-optimization/119960
	* tree-vect-slp.cc (vect_schedule_slp_node): Sanity
	check dominance check on operand defs.