tree-optimization/122723 - fix multi-step mask conversion

tree-optimization/122723 - fix multi-step mask conversion

The following fixes the logic in multi-step mask conversions with
respect to the case where we need to resort to the sbool variant
of the unpack optabs.  We were testing a wrong condition on when
that would apply in the multi-step case.

	PR tree-optimization/122723
	* tree-vect-stmts.cc (supportable_widening_operation):
	Correct optab to query in the multi-step case when it is
	supposed to hit the sbool case.