Add basic helpers for doing tests on the vars related optimization
passes.  The main goal is to lower the barrier to create tests during
development and debugging of the passes.  Full coverage is not a
requirement.

v2: Make find_next_intrinsic() skip blocks before 'after'. (Jason)
    Move nir_imm_ivec2() to nir_builder.h. (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>