[DAGCombine] Replace std::monostate equivalent in DAGCombiner.cpp

Remove the `UnitT` type and operators in favor of using `std::monostate`
directly.

Differential Revision: https://reviews.llvm.org/D131778

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -72,6 +72,7 @@
 #include <string>
 #include <tuple>
 #include <utility>
+#include <variant>
 
 using namespace llvm;
 
@@ -24943,13 +24944,6 @@ SDValue DAGCombiner::FindBetterChain(SDNode *N, SDValue OldChain) {
   return DAG.getTokenFactor(SDLoc(N), Aliases);
 }
 
-namespace {
-// TODO: Replace with with std::monostate when we move to C++17.
-struct UnitT { } Unit;
-bool operator==(const UnitT &, const UnitT &) { return true; }
-bool operator!=(const UnitT &, const UnitT &) { return false; }
-} // namespace
-
 // This function tries to collect a bunch of potentially interesting
 // nodes to improve the chains of, all at once. This might seem
 // redundant, as this function gets called when visiting every store
@@ -24970,8 +24964,8 @@ bool DAGCombiner::parallelizeChainedStores(StoreSDNode *St) {
   // the common case, every store writes to the immediately previous address
   // space and thus merged with the previous interval at insertion time.
 
-  using IMap =
-      llvm::IntervalMap<int64_t, UnitT, 8, IntervalMapHalfOpenInfo<int64_t>>;
+  using IMap = llvm::IntervalMap<int64_t, std::monostate, 8,
+                                 IntervalMapHalfOpenInfo<int64_t>>;
   IMap::Allocator A;
   IMap Intervals(A);
 
@@ -24998,7 +24992,8 @@ bool DAGCombiner::parallelizeChainedStores(StoreSDNode *St) {
     return false;
 
   // Add ST's interval.
-  Intervals.insert(0, (St->getMemoryVT().getSizeInBits() + 7) / 8, Unit);
+  Intervals.insert(0, (St->getMemoryVT().getSizeInBits() + 7) / 8,
+                   std::monostate{});
 
   while (StoreSDNode *Chain = dyn_cast<StoreSDNode>(STChain->getChain())) {
     if (Chain->getMemoryVT().isScalableVector())
@@ -25027,7 +25022,7 @@ bool DAGCombiner::parallelizeChainedStores(StoreSDNode *St) {
     // If there's a previous interval, we should start after it.
     if (I != Intervals.begin() && (--I).stop() <= Offset)
       break;
-    Intervals.insert(Offset, Offset + Length, Unit);
+    Intervals.insert(Offset, Offset + Length, std::monostate{});
 
     ChainedStores.push_back(Chain);
     STChain = Chain;