[PM] Make PreservedAnalyses::preserved take its parameter by const ref.

[jlebar]

Summary

Previously we were unnecessarily copying the argument.

[silvas]

Feel free to commit this kind of cleanup with post-commit review.

llvm/trunk/include/llvm/IR/PassManager.h

public:
   /// \brief Query whether an abstract pass ID is marked as preserved by this
   /// set.
   bool preserved(AnalysisKey *ID) const {
     return PreservedAnalysisIDs.count(&AllAnalysesKey) ||
            PreservedAnalysisIDs.count(ID);
   }
 
   /// \brief Query whether all of the analyses in the set are preserved.
-  bool preserved(PreservedAnalyses Arg) {
+  bool preserved(const PreservedAnalyses& Arg) {
     if (Arg.areAllPreserved())
       return areAllPreserved();
     for (auto ID : Arg.PreservedAnalysisIDs)
       if (!preserved(ID))
         return false;
     return true;
   }