nir/copy_prop_vars: change test helper to get intrinsics

nir/copy_prop_vars: change test helper to get intrinsics

Replace find_next_intrinsic(intrinsic, after) with
get_intrinsic(intrinsic, index).  This makes slightly more convenient
to check the resulting loads/stores/copies, since in most tests we
know which one we care about.  The cost is to perform more traversals,
but for such tests this is not a problem.

Added the ASSERT_EQ() on count to some tests missing it, so the
indices queried are always expected to find something.

Also, drop two nir_print_shader leftover calls in a test.

v2: Remove redundant assertions.  nir_src_comp_as_uint already
    assert what we need.  (Jason)

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