[Assignment Tracking] Check getTypeSizeInBits result for scalable vector types

Without this patch, in `getAssignmentInfo` the result of `getTypeSizeInBits` is
cast to `uint64_t`, which a) is an operation that will eventually be
unsupported by the API according to the comments, and b) causes an assertion
failure if the type is a scalable vector. Don't cast the `TypeSize` to
`uint64_t` and check `isScalable` before getting the fixed size.

This can result in incorrect variable locations, see llvm.org/PR62346 (but is
better than crashing).

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D149137