[X86][MC] Assert unexpected form in emitREXPrefix, NFCI

1. Add a variable `HasRegOp` to record if the instruction has a register operand
2. Enumerate all the formats with a register operand in the switch
2. Add a default (unreachable) label in the switch (suggested by @reames)

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D144776