[TableGen] Handle (outs variable_ops)

[dantrushin]

Summary

When variable_ops is specified in InOperandList of instruction,
it behaves as expected, i.e., does not count as operand.
So for (ins variable_ops) instruction description will have 0
operands. However when used in OutOperandList it is counted as
operand. So (outs variable_ops) results in instruction with
one def.
This patch makes behavior of variable_ops in out list to match
that of in list.

[dantrushin]

Add small but most important lost part.

[reames]

LGTM

llvm/utils/TableGen/CodeGenInstruction.cpp

CGIOperandList::CGIOperandList(Record *R) : TheDef(R) {
   } else
     PrintFatalError(R->getLoc(),
                     R->getName() + ": invalid input list: use 'ins'");
 
   unsigned MIOperandNo = 0;
   std::set<std::string> OperandNames;
   unsigned e = InDI->getNumArgs() + OutDI->getNumArgs();
   OperandList.reserve(e);
+  bool VariadicOuts = false;
   for (unsigned i = 0; i != e; ++i){
     Init *ArgInit;
     StringRef ArgName;
     if (i < NumDefs) {
       ArgInit = OutDI->getArg(i);
       ArgName = OutDI->getArgNameStr(i);
     } else {
       ArgInit = InDI->getArg(i-NumDefs);
if (Rec->isSubClassOf("RegisterOperand")) {
       if (unsigned NumArgs = MIOpInfo->getNumArgs())
         NumOps = NumArgs;
 
       if (Rec->isSubClassOf("PredicateOp"))
         isPredicable = true;
       else if (Rec->isSubClassOf("OptionalDefOperand"))
         hasOptionalDef = true;
     } else if (Rec->getName() == "variable_ops") {
+      if (i < NumDefs)
+        VariadicOuts = true;
       isVariadic = true;
       continue;
     } else if (Rec->isSubClassOf("RegisterClass")) {
       OperandType = "OPERAND_REGISTER";
     } else if (!Rec->isSubClassOf("PointerLikeRegClass") &&
                !Rec->isSubClassOf("unknown_class"))
       PrintFatalError(R->getLoc(), "Unknown operand class '" + Rec->getName() +
                                        "' in '" + R->getName() +
for (unsigned i = 0; i != e; ++i){
 
     OperandList.emplace_back(
         Rec, std::string(ArgName), std::string(PrintMethod),
         std::string(EncoderMethod), OperandNamespace + "::" + OperandType,
         MIOperandNo, NumOps, MIOpInfo);
     MIOperandNo += NumOps;
   }
 
+  if (VariadicOuts)
+    --NumDefs;
 
   // Make sure the constraints list for each operand is large enough to hold
   // constraint info, even if none is present.
   for (OperandInfo &OpInfo : OperandList)
     OpInfo.Constraints.resize(OpInfo.MINumOperands);
 }