[Alignment][NFC] Use llvm::Align for TargetLowering::getPrefLoopAlignment

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

Reviewed By: courbet

Subscribers: wuzish, arsenm, nemanjai, jvesely, nhaehnle, hiraditya, kbarton, MaskRay, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67386

llvm-svn: 371511