[TableGen] Add support for DefaultMode in per-HwMode encode/decode. (#83029)

Currently the decoder and encoder emitters will crash if DefaultMode is
used within an EncodingByHwMode. As can be done today for
RegInfoByHwMode and ValueTypeByHwMode, this patch adds support for this
usage in EncodingByHwMode:
  let EncodingInfos =
    EncodingByHwMode<[ModeA, DefaultMode], [EncA, EncDefault]>;