Negative loop increment?

Hi,

Does loopy support scheduling to negative loop increment? I tried the following:
```python
import numpy as np
import pyopencl as cl
import pyopencl.array
import pyopencl.clrandom

import loopy as lp

lp.set_caching_enabled(False)
from loopy.version import LOOPY_USE_LANGUAGE_VERSION_2018_2

from warnings import filterwarnings, catch_warnings

filterwarnings('error', category=lp.LoopyWarning)

ctx = cl.create_some_context(interactive=True)
queue = cl.CommandQueue(ctx)

n = 1000
a_bk = cl.array.arange(queue, n, dtype=np.float32)
a = a_bk.copy()

knl = lp.make_kernel(
"{ [i]: 0<=i