[TableGen] Fix a couple of crashes related to sub-operand dags

The added tests used to crash when attempting to dereference a nullptr
MIOpInfo or call MIOpInfo->getArg(0) on an empty MIOpInfo dag.

Author: s-barannikov

llvm/test/TableGen/FixedLenDecoderEmitter/sub-arg-dag-error-1.td

@@ -0,0 +1,28 @@
+// RUN: not llvm-tblgen -gen-disassembler -I %p/../../../include %s 2>&1 \
+// RUN:   | FileCheck %s --implicit-check-not=error:
+
+include "llvm/Target/Target.td"
+
+def R0 : Register<"r0">;
+def RC : RegisterClass<"MyTarget", [i32], 32, (add R0)>;
+
+// Used to crash.
+// CHECK: error: In instruction 'I', operand #0 has 1 sub-arg names, but no sub-operands
+
+def I : Instruction {
+  let Size = 1;
+  bits<8> Inst;
+  bits<1> r;
+
+  let Inst{0} = 0;
+  let Inst{1} = r;
+
+  let OutOperandList = (outs);
+  let InOperandList = (ins (RC $r):$op);
+}
+
+def II : InstrInfo;
+
+def MyTarget : Target {
+  let InstructionSet = II;
+}

llvm/test/TableGen/FixedLenDecoderEmitter/sub-arg-dag-error-2.td

@@ -0,0 +1,27 @@
+// RUN: not llvm-tblgen -gen-disassembler -I %p/../../../include %s 2>&1 \
+// RUN:   | FileCheck %s --implicit-check-not=error:
+
+include "llvm/Target/Target.td"
+
+def CustomOp : Operand<i32>;
+
+// Used to crash.
+// CHECK: error: In instruction 'I', operand #0 has 1 sub-arg names, expected 0
+
+def I : Instruction {
+  let Size = 1;
+  bits<8> Inst;
+  bits<1> i;
+
+  let Inst{0} = 0;
+  let Inst{1} = i;
+
+  let OutOperandList = (outs);
+  let InOperandList = (ins (CustomOp $i):$op);
+}
+
+def II : InstrInfo;
+
+def MyTarget : Target {
+  let InstructionSet = II;
+}

llvm/utils/TableGen/Common/CodeGenInstruction.cpp

@@ -143,12 +143,20 @@ CGIOperandList::CGIOperandList(const Record *R) : TheDef(R) {
         MIOperandNo, NumOps, MIOpInfo);
 
     if (SubArgDag) {
-      if (SubArgDag->getNumArgs() != NumOps) {
+      if (!MIOpInfo) {
         PrintFatalError(R->getLoc(), "In instruction '" + R->getName() +
                                          "', operand #" + Twine(i) + " has " +
                                          Twine(SubArgDag->getNumArgs()) +
-                                         " sub-arg names, expected " +
-                                         Twine(NumOps) + ".");
+                                         " sub-arg names, but no sub-operands");
+      }
+
+      unsigned NumSubArgs = SubArgDag->getNumArgs();
+      unsigned NumSubOps = MIOpInfo->getNumArgs();
+      if (NumSubArgs != NumSubOps) {
+        PrintFatalError(R->getLoc(),
+                        "In instruction '" + R->getName() + "', operand #" +
+                            Twine(i) + " has " + Twine(NumSubArgs) +
+                            " sub-arg names, expected " + Twine(NumSubOps));
       }
 
       for (unsigned j = 0; j < NumOps; ++j) {