[Alignment][NFC] Use Align with CreateMaskedScatter/Gather

[Alignment][NFC] Use Align with CreateMaskedScatter/Gather

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

This patch shows that CreateMaskedScatter/CreateMaskedGather can only take positive non zero alignment values.

Reviewers: courbet

Subscribers: hiraditya, llvm-commits, delena

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73361