Fix an EscapeAnalysis assert to handle recent changes.

setPointsToEdge should assert that its target isn't already merged,
but now that we batch up multiple merge requests, it's fine to allow
the target to be scheduled-for-merge.

Many assertions have been recently added and tightened in order to
"discover" unexpected cases. There's nothing incorrect about how these
cases were handled, but they lack unit tests. In this case I still
haven't been able to reduce a test case. I'm continuing to work on
it, but don't want to further delay the fix.

Author: atrick

include/swift/SILOptimizer/Analysis/EscapeAnalysis.h

@@ -449,7 +449,7 @@ class EscapeAnalysis : public BottomUpIPAnalysis {
 
     /// Sets the outgoing points-to edge. The \p To node must be a Content node.
     void setPointsToEdge(CGNode *To) {
-      assert(!To->mergeTo);
+      assert(!To->isMerged);
       assert(To->Type == NodeType::Content &&
              "Wrong node type for points-to edge");
       pointsToIsEdge = true;