[Alignment][NFC] Convert MIR Yaml to MaybeAlign

Summary:
Although it may look like non NFC it is. especially the MIRParser may set `0` to the MachineFrameInfo and MachineFunction, but they all deal with `Align` internally and assume that `0` means `1`.
https://github.com/llvm/llvm-project/blob/93fc0ba145c2f9619d3174c95cacda5d4a0a2815/llvm/include/llvm/CodeGen/MachineFrameInfo.h#L483

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/D77203