[NFC][TableGen] Refactor DecoderEmitter.cpp

- Add helper functions to insert ULEB128 encoded value and NumToSkip.
- Use ArrayRef<> instead of const vector references as function
  arguments.
- Return `OpHasCompleteDecoder` by value instead of by reference.
- Use range for loops.
- Remove {} around single line if/else bodies.
- In `emitSoftFailTableEntry`, unconditionally emit the Positive and
  Negative mask values, instead of explicitly emitting a 0 byte when
  the mask is not needed.

Author: jurahul