[Alignment][NFC] VectorLayout now uses Align internally

By rewritting `ScalarizerVisitor::getVectorLayout` in such a way it returns `VectorLayout` (or `None`) it becomes obvious that `VectorLayout::VecAlign` cannot be `0`.

This 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

Differential Revision: https://reviews.llvm.org/D82981