[SelectionDAG] Split very large token factors for chained stores to 64k chunks.

[fhahn]

Summary

Similar to D55073. Without this change, the DAG combiner crashes on code
with more than 64k of stores in a single basic block that form parallelizable
chains.

No test case, as it would be very IR file.

[fhahn]

update to use getTokenFactor.

[RKSimon]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

bool DAGCombiner::parallelizeChainedStores(StoreSDNode *St) {
   // the beginning to keep relative order consistent with FindBetterChains.
   auto hasImprovedChain = [&](SDValue ST) -> bool {
     return ST->getOperand(0) != NewChain;
   };
   bool AddNewChain = llvm::all_of(TFOps, hasImprovedChain);
   if (AddNewChain)
     TFOps.insert(TFOps.begin(), NewChain);
 
+  DAG.splitAcrossTokenFactors(SDLoc(STChain), TFOps);
   SDValue TF = DAG.getNode(ISD::TokenFactor, SDLoc(STChain), MVT::Other, TFOps);
   CombineTo(St, TF);
 
   AddToWorklist(STChain);
   // Add TF operands worklist in reverse order.
   for (auto I = TF->getNumOperands(); I;)
     AddToWorklist(TF->getOperand(--I).getNode());
   AddToWorklist(TF.getNode());