[clang-tidy] altera-id-dependent-backward-branch: refactor test

This is a preparation for future fixes that need more tests.

* Put all "Inferred Assignments" testing at the end
* Group together ID-dependent variable/member testing
* Group together unused inferred assignments
* Use a literal instead of `get_local_size` which may have special meaning
* Create a new `struct` for each variable because analysis is done per field,
  not per field per instance.

Depends on D145303

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D145304