Allow creating truncstores if custom

[arsenm]

Summary

I want to be able to custom lower some stores to illegal types, but currently the truncstore isn't created unless it is legal

[arsenm]

Ping

include/llvm/Target/TargetLowering.h

       getTruncStoreAction(ValVT.getSimpleVT(), MemVT.getSimpleVT()) == Legal;
   }
 
+  bool isTruncStoreLegalOrCustom(EVT ValVT, EVT MemVT) const {
+    if (!isTypeLegal(ValVT) || !MemVT.isSimple())
+      return false;
+
+    LegalizeAction Action = getTruncStoreAction(ValVT.getSimpleVT(),
+                                                MemVT.getSimpleVT());
+    return Action == Legal || Action == Custom;
+  }
+
   /// Return how the indexed load should be treated: either it is legal, needs
   /// to be promoted to a larger size, needs to be expanded to some other code
   /// sequence, or the target has a custom expander for it.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

   // truncating store.  We can do this even if this is already a truncstore.
   if ((Value.getOpcode() == ISD::FP_ROUND || Value.getOpcode() == ISD::TRUNCATE)
       && Value.getNode()->hasOneUse() && ST->isUnindexed() &&
-      TLI.isTruncStoreLegal(Value.getOperand(0).getValueType(),
+      TLI.isTruncStoreLegalOrCustom(Value.getOperand(0).getValueType(),
                                     ST->getMemoryVT())) {
     return DAG.getTruncStore(Chain, SDLoc(N), Value.getOperand(0),
                              Ptr, ST->getMemoryVT(), ST->getMemOperand());
   }