SIL Optimizer: Fix bug in EscapeAnalysis::isReachable()

We weren't clearing the worklist flags if returning true here. Oops!

This would manifest as alias analysis returning different results
for the same operands over time, which confused ARC code motion
into dropping release instructions.

Author: slavapestov

lib/SILOptimizer/Analysis/EscapeAnalysis.cpp

@@ -561,8 +561,10 @@ bool EscapeAnalysis::ConnectionGraph::isReachable(CGNode *From, CGNode *To) {
   From->isInWorkList = true;
   for (unsigned Idx = 0; Idx < WorkList.size(); ++Idx) {
     CGNode *Reachable = WorkList[Idx];
-    if (Reachable == To)
+    if (Reachable == To) {
+      clearWorkListFlags(WorkList);
       return true;
+    }
     for (Predecessor Pred : Reachable->Preds) {
       CGNode *PredNode = Pred.getPointer();
       if (!PredNode->isInWorkList) {