[ValueTracking] Fix usage of DataLayout::getTypeStoreSize()

Summary:
DataLayout::getTypeStoreSize() returns TypeSize.

For cases where it can not be scalable vector (e.g., GlobalVariable),
explicitly call TypeSize::getFixedSize().

For cases where scalable property doesn't matter, (e.g., check for
zero-sized type), use TypeSize::isNonZero().

Reviewers: sdesmalen, efriedma, apazos, reames

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76454