TableGen: avoid dereferencing nullptr variable

ARM64 ended up reaching odder parts of TableGen alias generation than
current backends and caused a segfault.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205089 91177308-0d34-0410-b5e6-96231b3b80d8

Author: TNorthover

utils/TableGen/CodeGenInstruction.cpp

@@ -436,26 +436,33 @@ bool CodeGenInstAlias::tryAliasOpMatch(DagInit *Result, unsigned AliasOpNo,
                                        ResultOperand &ResOp) {
   Init *Arg = Result->getArg(AliasOpNo);
   DefInit *ADI = dyn_cast<DefInit>(Arg);
+  Record *ResultRecord = ADI ? ADI->getDef() : 0;
 
   if (ADI && ADI->getDef() == InstOpRec) {
     // If the operand is a record, it must have a name, and the record type
     // must match up with the instruction's argument type.
     if (Result->getArgName(AliasOpNo).empty())
       PrintFatalError(Loc, "result argument #" + utostr(AliasOpNo) +
                     " must have a name!");
-    ResOp = ResultOperand(Result->getArgName(AliasOpNo), ADI->getDef());
+    ResOp = ResultOperand(Result->getArgName(AliasOpNo), ResultRecord);
     return true;
   }
 
   // For register operands, the source register class can be a subclass
   // of the instruction register class, not just an exact match.
+  if (InstOpRec->isSubClassOf("RegisterOperand"))
+    InstOpRec = InstOpRec->getValueAsDef("RegClass");
+
+  if (ADI && ADI->getDef()->isSubClassOf("RegisterOperand"))
+    ADI = ADI->getDef()->getValueAsDef("RegClass")->getDefInit();
+
   if (ADI && ADI->getDef()->isSubClassOf("RegisterClass")) {
     if (!InstOpRec->isSubClassOf("RegisterClass"))
       return false;
     if (!T.getRegisterClass(InstOpRec)
               .hasSubClass(&T.getRegisterClass(ADI->getDef())))
       return false;
-    ResOp = ResultOperand(Result->getArgName(AliasOpNo), ADI->getDef());
+    ResOp = ResultOperand(Result->getArgName(AliasOpNo), ResultRecord);
     return true;
   }
 
@@ -468,9 +475,6 @@ bool CodeGenInstAlias::tryAliasOpMatch(DagInit *Result, unsigned AliasOpNo,
       InstOpRec = cast<DefInit>(DI->getArg(0))->getDef();
     }
 
-    if (InstOpRec->isSubClassOf("RegisterOperand"))
-      InstOpRec = InstOpRec->getValueAsDef("RegClass");
-
     if (!InstOpRec->isSubClassOf("RegisterClass"))
       return false;
 
@@ -484,7 +488,7 @@ bool CodeGenInstAlias::tryAliasOpMatch(DagInit *Result, unsigned AliasOpNo,
       PrintFatalError(Loc, "result fixed register argument must "
                       "not have a name!");
 
-    ResOp = ResultOperand(ADI->getDef());
+    ResOp = ResultOperand(ResultRecord);
     return true;
   }