[GlobalISel] NFC: Have LLT::getSizeInBits/Bytes return a TypeSize.

To reflect that the size may be scalable, a TypeSize is returned
instead of an unsigned. In places where the result is used,
it currently relies on an implicit cast of TypeSize -> uint64_t,
which asserts that the type is not scalable.

This patch is NFC for fixed-width vectors.

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D104454