[X86] Not track size of the boudaryalign fragment during the layout

Summary:
Currently the boundaryalign fragment caches its size during the process
of layout and then it is relaxed and update the size in each iteration. This
behaviour is unnecessary and ugly.

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

Reviewed By: MaskRay

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75404