[CtxProf] Fix warnings

This patch fixes:

  llvm/lib/Transforms/Instrumentation/PGOCtxProfFlattening.cpp:214:14:
  error: unused variable 'Index' [-Werror,-Wunused-variable]

  llvm/lib/Transforms/Instrumentation/PGOCtxProfFlattening.cpp:284:6:
  error: unused function 'areAllBBsReachable'
  [-Werror,-Wunused-function]

Author: kazutakahirata

llvm/lib/Transforms/Instrumentation/PGOCtxProfFlattening.cpp

@@ -216,6 +216,7 @@ class ProfileAnnotator final {
                "The index must be inside the counters vector by construction - "
                "tripping this assertion indicates a bug in how the contextual "
                "profile is managed by IPO transforms");
+        (void)Index;
         Count = Counters[Ins->getIndex()->getZExtValue()];
       }
       auto [It, Ins] =
@@ -280,7 +281,8 @@ class ProfileAnnotator final {
   }
 };
 
-bool areAllBBsReachable(const Function &F, FunctionAnalysisManager &FAM) {
+[[maybe_unused]] bool areAllBBsReachable(const Function &F,
+                                         FunctionAnalysisManager &FAM) {
   auto &DT = FAM.getResult<DominatorTreeAnalysis>(const_cast<Function &>(F));
   return llvm::all_of(
       F, [&](const BasicBlock &BB) { return DT.isReachableFromEntry(&BB); });