[X86][MC] Disable Prefix padding after hardcode/prefix

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

Reviewed By: craig.topper

Subscribers: hiraditya, llvm-commits, annita.zhang

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76475