[X86][MC] Fix the bug for prefix padding support

Summary:
There is a tiny logic error of D75300, making branch is not
correctly aligned with option -x86-pad-max-prefix-size

Reviewers: reames, MaskRay, craig.topper, LuoYuanke, jyknight

Reviewed By: reames

Subscribers: hiraditya, llvm-commits, annita.zhang

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76285