nir/tests: Add copy propagation tests with scoped_memory_barrier

nir/tests: Add copy propagation tests with scoped_memory_barrier

Three groups of tests, effectively defining what cases the
optimization is allowed or prevented

- Redudant loads       (a load  generated the value)
- Propagate SSA values (a store generated the value)
- Propagate a var      (a copy  generated the value)

Change the shader type of the tests to be COMPUTE so
nir_var_mem_shared can also be used.  Doesn't affect the semantic of
the copy propagation.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>