[X86][MC] Use MCInstrDesc to access CondCode operand

BOLT may add an extra MCOperand as annotation, making the number of operands in
MCInst different from MCInstrDesc information. A recent change broke our use as
the last operand is no longer a cond code.

Partially revert 287dd014 to rely on MCInstrDesc
to access CondCode operand.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D151930