[analyzer] Minor optimization: avoid setting state if unchanged

[ddcc]

Summary

Split out optimization from D26061

[dcoughlin]

LGTM. Please commit!

lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

ConditionTruthVal RangeConstraintManager::checkNull(ProgramStateRef State,
   return ConditionTruthVal();
 }
 
 /// Scan all symbols referenced by the constraints. If the symbol is not alive
 /// as marked in LSymbols, mark it as dead in DSymbols.
 ProgramStateRef
 RangeConstraintManager::removeDeadBindings(ProgramStateRef state,
                                            SymbolReaper& SymReaper) {
-
+  bool Changed = false;
   ConstraintRangeTy CR = state->get<ConstraintRange>();
-  ConstraintRangeTy::Factory& CRFactory = state->get_context<ConstraintRange>();
+  ConstraintRangeTy::Factory &CRFactory = state->get_context<ConstraintRange>();
 
   for (ConstraintRangeTy::iterator I = CR.begin(), E = CR.end(); I != E; ++I) {
     SymbolRef sym = I.getKey();
-    if (SymReaper.maybeDead(sym))
+    if (SymReaper.maybeDead(sym)) {
+      Changed = true;
       CR = CRFactory.remove(CR, sym);
     }
+  }
 
-  return state->set<ConstraintRange>(CR);
+  return Changed ? state->set<ConstraintRange>(CR) : state;
 }
 
 RangeSet
 RangeConstraintManager::GetRange(ProgramStateRef state, SymbolRef sym) {
   if (ConstraintRangeTy::data_type* V = state->get<ConstraintRange>(sym))
     return *V;
 
   // Lazily generate a new RangeSet representing all possible values for the