[DAG] mergeTruncStores - early out if we collect more than the maximum number of stores

If we have an excessive number of stores in a single chain then the candidate WideVT may exceed the maximum width of an EVT integer type (and will assert) - but since mergeTruncStores doesn't support anything wider than a i64 store we should just early-out if we've collected more than stores than that.

Fixes llvm#63306

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -8679,9 +8679,12 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
       !N->isSimple() || N->isIndexed())
     return SDValue();
 
-  // Collect all of the stores in the chain.
+  // Collect all of the stores in the chain, upto the maximum store width (i64).
   SDValue Chain = N->getChain();
   SmallVector<StoreSDNode *, 8> Stores = {N};
+  unsigned NarrowNumBits = MemVT.getScalarSizeInBits();
+  unsigned MaxWideNumBits = 64;
+  unsigned MaxStores = MaxWideNumBits / NarrowNumBits;
   while (auto *Store = dyn_cast<StoreSDNode>(Chain)) {
     // All stores must be the same size to ensure that we are writing all of the
     // bytes in the wide value.
@@ -8695,6 +8698,8 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
       return SDValue();
     Stores.push_back(Store);
     Chain = Store->getChain();
+    if (MaxStores < Stores.size())
+      return SDValue();
   }
   // There is no reason to continue if we do not have at least a pair of stores.
   if (Stores.size() < 2)
@@ -8703,7 +8708,6 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
   // Handle simple types only.
   LLVMContext &Context = *DAG.getContext();
   unsigned NumStores = Stores.size();
-  unsigned NarrowNumBits = N->getMemoryVT().getScalarSizeInBits();
   unsigned WideNumBits = NumStores * NarrowNumBits;
   EVT WideVT = EVT::getIntegerVT(Context, WideNumBits);
   if (WideVT != MVT::i16 && WideVT != MVT::i32 && WideVT != MVT::i64)