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

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.

Differential Revision: https://reviews.llvm.org/D123238

Author: kovdan01

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -280,6 +280,11 @@ class SelectionDAG {
 
   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?

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -615,8 +615,10 @@ END_TWO_BYTE_PACK()
   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;
 
   //===--------------------------------------------------------------------===//