Ignore register mask operands when lowering instructions to MC.

This is similar to implicit register operands.  MC doesn't understand
register liveness and call clobbers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148437 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Mips/MipsMCInstLower.cpp

@@ -192,6 +192,8 @@ MCOperand MipsMCInstLower::LowerOperand(const MachineOperand& MO,
   case MachineOperand::MO_ConstantPoolIndex:
   case MachineOperand::MO_BlockAddress:
     return LowerSymbolOperand(MO, MOTy, offset);
+  case MachineOperand::MO_RegisterMask:
+    break;
  }
 
   return MCOperand();