[Alignment] Use Align for TFI.getStackAlignment() in X86ISelLowering

Summary:
This is patch is part of a series to introduce an Alignment type.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html
See this patch for the introduction of the type: https://reviews.llvm.org/D64790

Reviewers: courbet, craig.topper, rnk

Reviewed By: rnk

Subscribers: rnk, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69034