[NFC] BasicBlockWorklist: Add ::getVisited.

Expose the BasicBlockSet of visited blocks for use by code responsible
for testing set membership.  Return only a const reference so that
clients can't easily mess up the worklist.

Author: nate-chandler

include/swift/SIL/BasicBlockDatastructures.h

@@ -123,6 +123,8 @@ class BasicBlockWorklist {
 
   /// Returns true if \p block was visited, i.e. has been added to the worklist.
   bool isVisited(SILBasicBlock *block) const { return visited.contains(block); }
+
+  BasicBlockSet const &getVisited() const { return visited; }
 };