[NFC][DecoderEmitter] Code cleanup in `DecoderEmitter::emitTable` (#158014)
Several code cleanup changes in code to emit decoder tables:
- Start comments on each line at a fixed column for readibility.
- Combine repeated code to decode and emit ULEB128 into a single
function.
- Add helper `getDecoderOpName` to print decoder op.
- Print Filter/CheckField/predicate index values with those opcodes.