nir/loop_analyze: Add basic unit test framework

This test comes from a comment in the loop analysis code.

The ine_zero test checks that zero iteration loops involving ine are
correctly identified.

v2: Add ine_zero test. Suggested by Tim.

v3: Rename test cases.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19732>