[SelectionDAG] Use LLVM_ENABLE_ABI_BREAKING_CHECKS to save space

[davide]

Summary

As recommended by Duncan's post commit review on r251209.

include/llvm/CodeGen/SelectionDAG.h

class SelectionDAG {
   BumpPtrAllocator OperandAllocator;
 
   /// Pool allocation for misc. objects that are created once per SelectionDAG.
   BumpPtrAllocator Allocator;
 
   /// Tracks dbg_value information through SDISel.
   SDDbgInfo *DbgInfo;
 
+#ifndef LLVM_ENABLE_ABI_BREAKING_CHECKS
   uint16_t NextPersistentId = 0;
+#endif
 
 public:
   /// Clients of various APIs that cause global effects on
   /// the DAG can optionally implement this interface.  This allows the clients
   /// to handle the various sorts of updates that happen.
   ///
   /// A DAGUpdateListener automatically registers itself with DAG when it is
   /// constructed, and removes itself when destroyed in RAII fashion.

include/llvm/CodeGen/SelectionDAGNodes.h

private:
 
   /// Return a pointer to the specified value type.
   static const EVT *getValueTypeList(EVT VT);
 
   friend class SelectionDAG;
   friend struct ilist_traits<SDNode>;
 
 public:
+#ifndef LLVM_ENABLE_ABI_BREAKING_CHECKS
   /// Unique and persistent id per SDNode in the DAG.
   /// Used for debug printing.
   uint16_t PersistentId;
+#endif
 
   //===--------------------------------------------------------------------===//
   //  Accessors
   //
 
   /// Return the SelectionDAG opcode value for this node. For
   /// pre-isel nodes (those for which isMachineOpcode returns false), these
   /// are the opcode values in the ISD and <target>ISD namespaces. For