Compute the loop nest depth of each CfgNode and weight Variables by it.

src/IceInst.cpp

Index: src/IceInst.cpp
diff --git a/src/IceInst.cpp b/src/IceInst.cpp
index 4b89eb77b0fa88f5f692f637a5ef892305b1a62b..ebbf89550376ce0ca2678aa158c58b080bcb56fa 100644
--- a/src/IceInst.cpp
+++ b/src/IceInst.cpp
@@ -432,6 +432,10 @@ InstStore::InstStore(Cfg *Func, Operand *Data, Operand *Addr)
   addSource(Data);
 }
 
+Variable *InstStore::getRmwBeacon() const {
+  return llvm::dyn_cast<Variable>(getSrc(2));
+}
+
 void InstStore::setRmwBeacon(Variable *Beacon) {
   Dest = llvm::dyn_cast<Variable>(getData());
   Srcs[2] = Beacon;