[llvm][MemProf] Fix unused variable warning in release build

g++-13 warned that:
llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp:1645:8: warning: variable ‘PrevIterCreatedNode’ set but not used [-Wunused-but-set-variable]
 1645 |   bool PrevIterCreatedNode = false;
      |        ^~~~~~~~~~~~~~~~~~~

When asserts were not enabled.

Author: DavidSpickett

llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp

@@ -1642,7 +1642,7 @@ void CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::
   // this entry.
   DenseSet<uint32_t> LastNodeContextIds = LastNode->getContextIds();
 
-  bool PrevIterCreatedNode = false;
+  [[maybe_unused]] bool PrevIterCreatedNode = false;
   bool CreatedNode = false;
   for (unsigned I = 0; I < Calls.size();
        I++, PrevIterCreatedNode = CreatedNode) {