[DebugInfo] Add MI instruction 'DBG_LABEL label'.

[HsiangKai]

Summary

Add new MI opcode TargetOpcode::DBG_LABEL. The MachineInstr DBG_LABEL uses
DILabel metadata as its parameter. User could call MachineInstr::getDebugLabel()
to get DILabel metadata of the MachineInstr.

include/llvm/CodeGen/MachineInstr.h

public:
   /// Return the debug variable referenced by
   /// this DBG_VALUE instruction.
   const DILocalVariable *getDebugVariable() const;
 
   /// Return the complex address expression referenced by
   /// this DBG_VALUE instruction.
   const DIExpression *getDebugExpression() const;
 
+  /// Return the debug label referenced by
+  /// this DBG_LABEL instruction.
+  const DILabel *getDebugLabel() const;
+
   /// Emit an error referring to the source location of this instruction.
   /// This should only be used for inline assembly that is somehow
   /// impossible to compile. Other errors should have been handled much
   /// earlier.
   ///
   /// If this method returns, the caller should try to recover from the error.
   void emitError(StringRef Msg) const;
 
public:
   bool isCFIInstruction() const {
     return getOpcode() == TargetOpcode::CFI_INSTRUCTION;
   }
 
   // True if the instruction represents a position in the function.
   bool isPosition() const { return isLabel() || isCFIInstruction(); }
 
   bool isDebugValue() const { return getOpcode() == TargetOpcode::DBG_VALUE; }
+  bool isDebugLabel() const { return getOpcode() == TargetOpcode::DBG_LABEL; }
 
   /// A DBG_VALUE is indirect iff the first operand is a register and
   /// the second operand is an immediate.
   bool isIndirectDebugValue() const {
     return isDebugValue()
       && getOperand(0).isReg()
       && getOperand(1).isImm();
   }
bool isMetaInstruction() const {
     default:
       return false;
     case TargetOpcode::IMPLICIT_DEF:
     case TargetOpcode::KILL:
     case TargetOpcode::CFI_INSTRUCTION:
     case TargetOpcode::EH_LABEL:
     case TargetOpcode::GC_LABEL:
     case TargetOpcode::DBG_VALUE:
+    case TargetOpcode::DBG_LABEL:
     case TargetOpcode::LIFETIME_START:
     case TargetOpcode::LIFETIME_END:
       return true;
     }
   }
 
   /// Return true if this is a transient instruction that is either very likely
   /// to be eliminated during register allocation (such as copy-like

include/llvm/Support/TargetOpcodes.def

 /// virtual registers have been created for all the instructions, and it's
 /// only needed in cases where the register classes implied by the
 /// instructions are insufficient. It is emitted as a COPY MachineInstr.
   HANDLE_TARGET_OPCODE(COPY_TO_REGCLASS)
 
 /// DBG_VALUE - a mapping of the llvm.dbg.value intrinsic
 HANDLE_TARGET_OPCODE(DBG_VALUE)
 
+/// DBG_LABEL - a mapping of the llvm.dbg.label intrinsic
+HANDLE_TARGET_OPCODE(DBG_LABEL)
+
 /// REG_SEQUENCE - This variadic instruction is used to form a register that
 /// represents a consecutive sequence of sub-registers. It's used as a
 /// register coalescing / allocation aid and must be eliminated before code
 /// emission.
 // In SDNode form, the first operand encodes the register class created by
 // the REG_SEQUENCE, while each subsequent pair names a vreg + subreg index
 // pair.  Once it has been lowered to a MachineInstr, the regclass operand
 // is no longer present.

include/llvm/Target/Target.td

def COPY_TO_REGCLASS : StandardPseudoInstruction {
   let isAsCheapAsAMove = 1;
 }
 def DBG_VALUE : StandardPseudoInstruction {
   let OutOperandList = (outs);
   let InOperandList = (ins variable_ops);
   let AsmString = "DBG_VALUE";
   let hasSideEffects = 0;
 }
+def DBG_LABEL : StandardPseudoInstruction {
+  let OutOperandList = (outs);
+  let InOperandList = (ins variable_ops);
+  let AsmString = "DBG_LABEL";
+  let hasSideEffects = 0;
+}
 def REG_SEQUENCE : StandardPseudoInstruction {
   let OutOperandList = (outs unknown:$dst);
   let InOperandList = (ins unknown:$supersrc, variable_ops);
   let AsmString = "";
   let hasSideEffects = 0;
   let isAsCheapAsAMove = 1;
 }
 def COPY : StandardPseudoInstruction {

lib/CodeGen/MachineInstr.cpp

if (i + NumOps > OpIdx) {
         *GroupNo = Group;
       return i;
     }
     ++Group;
   }
   return -1;
 }
 
+const DILabel *MachineInstr::getDebugLabel() const {
+  assert(isDebugLabel() && "not a DBG_LABEL");
+  return cast<DILabel>(getOperand(0).getMetadata());
+}
+
 const DILocalVariable *MachineInstr::getDebugVariable() const {
   assert(isDebugValue() && "not a DBG_VALUE");
   return cast<DILocalVariable>(getOperand(2).getMetadata());
 }
 
 const DIExpression *MachineInstr::getDebugExpression() const {
   assert(isDebugValue() && "not a DBG_VALUE");
   return cast<DIExpression>(getOperand(3).getMetadata());