merge_loop_domains not correct with imperfectly nested reductions

loopy.loop.merge_loop_domains is still not fully correct: It will still merge imperfectly nested loops when the inner loop is a reduction, because it only considers full statements in the detection of imperfect nesting. A possible

This isn't immensely urgent because merge_loop_domains is not part of the public, documented API and only used in the Fortran frontend path, which does not generate reductions.