[CodeGen] Add lifetime markers to the list of meta-instructions.

[mattd]

Summary

Since the lifetime markers are metadata instructions, they should probably be treated as such by the isMetaInstruction predicate.
There was no issue that provoked this change, I just ran across it while investigating another issue.

[aprantl]

That seems reasonable.

llvm/trunk/include/llvm/CodeGen/MachineInstr.h

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::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
   /// instructions), or if this instruction doesn't have an execution-time cost.
   bool isTransient() const {