[Alignment][NFC] Return Align for SelectionDAGNodes::getOriginalAlignment/getAlignment

Summary:
Also deprecate getOriginalAlignment, getAlignment will take much more time as it is pervasive through the codebase (including TableGened files).

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

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76933