Remove inexplicable data in CallGraphAnalysis.

I suspect this is a holdover from when I rewrote most of the call graph
code. In any event, it's unused since we store this in the call graph
itself.

Swift SVN r27072

include/swift/SILAnalysis/CallGraphAnalysis.h

@@ -364,7 +364,6 @@ public:
 /// The Call Graph Analysis provides information about the call graph.
 class CallGraphAnalysis : public SILAnalysis {
   SILModule *M;
-  std::vector<SILFunction *> BottomUpFunctionOrder;
   CallGraph *CG;
 
 public:
@@ -394,7 +393,6 @@ public:
   virtual void invalidate(SILAnalysis::PreserveKind K) {
     if (K & PreserveKind::Calls) return;
 
-    BottomUpFunctionOrder.clear();
     delete CG;
     CG = nullptr;
   }