Array analysis failure cannot unpack non-iterable NoneType object

[stuartarchibald]

Reporting a bug

• I am using the latest released master version of Numba (most recent is visible in
  the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included below a minimal working reproducer (if you are unsure how
  to write one see http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports).

This:

from numba import njit, prange
import numpy as np

@njit(parallel=True)
def f3():
    arr = np.arange(20)
    for i in prange(2):
        for j in range(20):
            idx = j

        _ = arr[:idx]

f3()

does this:

numba/numba/parfors/array_analysis.py", line 826, in plus
    (var, offset) = x
TypeError: Failed in nopython mode pipeline (step: convert to parfors)
cannot unpack non-iterable NoneType object

[stuartarchibald]

CC @DrTodd13

[sklam]

Fixed by #5694