StackNesting: fix a corner case crash related to unreachable blocks

When deallocs are inserted at block boundaries, it's necessary to recompute the data flow.

rdar://problem/55249524

include/swift/SILOptimizer/Utils/StackNesting.h

@@ -155,10 +155,13 @@ private:
     return bitNumberForAlloc(AllocInst);
   }
 
+  /// Insert deallocations at block boundaries.
+  Changes insertDeallocsAtBlockBoundaries();
+
   /// Modifies the SIL to end up with a correct stack nesting.
   ///
   /// Returns the status of what changes were made.
-  Changes adaptDeallocs();
+  bool adaptDeallocs();
 };
 
 } // end namespace swift