[X86][MC][NFC] Refine code in X86MCCodeEmitter.cpp about opcode prefix

1. Make code clearer by separating the logic of setting bits from the
   logic of how a prefix is encoded
2. Extract common code into functions to avoid code duplication
3. Return a enum rather a boolean to ehance scalability and uniform
   the behavior of functions

Reviewed By: pengfei, craig.topper

Differential Revision: https://reviews.llvm.org/D143471