[TableGen] Add support for variable length instruction in decoder generator

To support variable length instructions, I think of them as fixed length instructions with the "maximum length". For example, if there're three instructions with 2, 6 and 9 bytes, we can fit them into the algorithm by treating them all as 9 bytes.

Also, since we can't know the length of the instruction in advance, there is a function object with type `void(APInt &, uint64_t)` added in the parameter list of `decodeInstruction` and `fieldFromInstruction`. We can use this to supply the additional bits the decoder needs after we know the opcode of the instruction.

Finally, `InstrLenTable` is added to let the decoder know the length of the instructions.

See D120960 for its usage.

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D120958