[DebugInfo] Convert intrinsic llvm.dbg.label to MachineInstr.

In order to convert LLVM IR to MachineInstr, we need a new TargetOpcode,
DBG_LABEL, to ‘lower’ intrinsic llvm.dbg.label. The patch
creates this new TargetOpcode and convert intrinsic llvm.dbg.label to
MachineInstr through SelectionDAG.

In SelectionDAG, debug information is stored in SDDbgInfo. We create a
new data member of SDDbgInfo for labels and use the new data member,
SDDbgLabel, to create DBG_LABEL MachineInstr.

The new DBG_LABEL MachineInstr uses label metadata from LLVM IR as its
parameter. So, the backend could get metadata information of labels from
DBG_LABEL MachineInstr.

Differential Revision: https://reviews.llvm.org/D45341

Patch by Hsiangkai Wang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331842 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/PatchableFunction.cpp

@@ -49,6 +49,7 @@ static bool doesNotGeneratecode(const MachineInstr &MI) {
   case TargetOpcode::EH_LABEL:
   case TargetOpcode::GC_LABEL:
   case TargetOpcode::DBG_VALUE:
+  case TargetOpcode::DBG_LABEL:
     return true;
   }
 }