[LV] Make the new getter return a const reference. NFC

LoopVectorizationLegality holds a constant reference to LAI, so this
will have to be const as well.

Also added missed function comment.

llvm-svn: 272851

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1448,7 +1448,9 @@ private:
     emitAnalysisDiag(TheFunction, TheLoop, *Hints, Message);
   }
 
-  ValueToValueMap &getSymbolicStrides() { return SymbolicStrides; }
+  /// \brief If an access has a symbolic strides, this maps the pointer value to
+  /// the stride symbol.
+  const ValueToValueMap &getSymbolicStrides() { return SymbolicStrides; }
 
   unsigned NumPredStores;