[GlobalISel] Add scalable property to LLT types.

This patch aims to add the scalable property to LLT. The rest of the
patch-series changes the interfaces to take/return ElementCount and
TypeSize, which both have the ability to represent the scalable property.

The changes are mostly mechanical and aim to be non-functional changes
for fixed-width vectors.

For scalable vectors some unit tests have been added, but no effort has
been put into making any of the GlobalISel algorithms work with scalable
vectors yet. That will be left as future work.

The work is split into a series of 5 patches to make reviews easier.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D104450