Make some DataLayout pointers const.

No functionality change. Just reduces the noise of an upcoming patch.

llvm-svn: 202087
diff --git a/llvm/lib/Transforms/Scalar/ConstantProp.cpp b/llvm/lib/Transforms/Scalar/ConstantProp.cpp
index c15217c..76815a4 100644
--- a/llvm/lib/Transforms/Scalar/ConstantProp.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstantProp.cpp
@@ -67,7 +67,7 @@
       WorkList.insert(&*i);
   }
   bool Changed = false;
-  DataLayout *DL = getAnalysisIfAvailable<DataLayout>();
+  const DataLayout *DL = getAnalysisIfAvailable<DataLayout>();
   TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
 
   while (!WorkList.empty()) {