Clean come dead code

These codes deleted are dead code, we never go into it.
1. In AggressiveAntiDepBreaker.cpp, have assert AntiDepReg != 0.
2. IfConversion.cpp, Kind can only be one unique value, so isFalse && isRev
    can never be true.
3. DAGCombiner.cpp, at line 3675, we have considered the condition like
```
  // fold (sub x, c) -> (add x, -c)
  if (N1C) {
    return DAG.getNode(ISD::ADD, DL, VT, N0,
                       DAG.getConstant(-N1C->getAPIntValue(), DL, VT));
  }
```
4. ScheduleDAGSDNodes.cpp, we have Latency > 1 at line 663
5. MasmParser.cpp, code exists in a switch-case block which decided by
    the value FirstTokenKind, at line 1621, FirstTokenKind could only be
    one of AsmToken::Dollar, AsmToken::At and AsmToken::Identifier.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D148610