tree-optimization/121756 - handle irreducible regions when sinkingHEADtrunkmaster

The sinking code currently does not heuristically avoid placing
code into an irreducible region in the same way it avoids placing
into a deeper loop nest.  Critically for the PR we may not insert
a VDEF into a irreducible region that does not contain a virtual
definition.  The following adds the missing heuristic and also
a stop-gap for the VDEF issue - since we cannot determine
validity inside an irreducible region we have to reject any
VDEF movement with destination inside such region, even when
it originates there.  In particular irreducible sub-cycles are
not tracked separately and can cause issues.

I chose to not complicate the already partly incomplete assert
but prune it down to essentials.

	PR tree-optimization/121756
	* tree-ssa-sink.cc (select_best_block): Avoid irreducible
	regions in otherwise same loop depth.
	(statement_sink_location): When sinking a VDEF, never place
	that into an irreducible region.

	* gcc.dg/torture/pr121756.c: New testcase.

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