[NFC]add & to avoid copy

[XinWang10]

Summary

The elements in FragmentMap are big objects, use reference can get
better performance, as someone do in line 1912.

[XinWang10]

rebase

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

std::sort(
           assert(!(Elmt.getFragmentOrDefault() == Next.getFragmentOrDefault()));
           return Elmt.getFragmentOrDefault().SizeInBits >
                  Next.getFragmentOrDefault().SizeInBits;
         });
   }
 
   // Build the map.
   AssignmentTrackingLowering::OverlapMap Map;
-  for (auto Pair : FragmentMap) {
+  for (auto &Pair : FragmentMap) {
     auto &Frags = Pair.second;
     for (auto It = Frags.begin(), IEnd = Frags.end(); It != IEnd; ++It) {
       DIExpression::FragmentInfo Frag = It->getFragmentOrDefault();
       // Find the frags that this is contained within.
       //
       // Because Frags is sorted by size and none have the same offset and
       // size, we know that this frag can only be contained by subsequent
       // elements.