[NFC][CodeGen] Add comments for SDNode debug ID declaration

[kovdan01]

Summary

Normally, we place fields serving for debug purpose declarations
under #if LLVM_ENABLE_ABI_BREAKING_CHECKS. For SDNode::PersistentId and
SelectionDAG::NextPersistentId, we do not want to do so because it adds
unneeded complexity without noticeable benefits (see discussion with @thakis
in D120714). This patch adds comments describing why we don't place those
fields under #if not to confuse anyone more.

llvm/include/llvm/CodeGen/SelectionDAG.h

class SelectionDAG {
   struct CallSiteDbgInfo {
     CallSiteInfo CSInfo;
     MDNode *HeapAllocSite = nullptr;
     bool NoMerge = false;
   };
 
   DenseMap<const SDNode *, CallSiteDbgInfo> SDCallSiteDbgInfo;
 
+  /// PersistentId counter to be used when inserting the next
+  /// SDNode to this SelectionDAG. We do not place that under
+  /// `#if LLVM_ENABLE_ABI_BREAKING_CHECKS` intentionally because
+  /// it adds unneeded complexity without noticeable
+  /// benefits (see discussion with @thakis in D120714).
   uint16_t NextPersistentId = 0;
 
   /// Are instruction referencing variable locations desired for this function?
   bool UseInstrRefDebugInfo = false;
 
 public:
   /// Clients of various APIs that cause global effects on
   /// the DAG can optionally implement this interface.  This allows the clients

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

private:
   DebugLoc debugLoc;
 
   /// Return a pointer to the specified value type.
   static const EVT *getValueTypeList(EVT VT);
 
   SDNodeFlags Flags;
 
 public:
-  /// Unique and persistent id per SDNode in the DAG.
-  /// Used for debug printing.
+  /// Unique and persistent id per SDNode in the DAG. Used for debug printing.
+  /// We do not place that under `#if LLVM_ENABLE_ABI_BREAKING_CHECKS`
+  /// intentionally because it adds unneeded complexity without noticeable
+  /// benefits (see discussion with @thakis in D120714).
   uint16_t PersistentId;
 
   //===--------------------------------------------------------------------===//
   //  Accessors
   //
 
   /// Return the SelectionDAG opcode value for this node. For
   /// pre-isel nodes (those for which isMachineOpcode returns false), these