ndarray.flat does not support setitem

[stuartarchibald]

Following on from #3117, setitem on flat does not work.

from numba import njit
import numpy as np
@njit
def flat(X, d):
    N = X.shape[0]
    X.flat[:N**2:N+1] = d

d = 10
X = np.ones((5, 5))
flat(X, d)

gives:

$ python foo2.py 
Traceback (most recent call last):
  File "<path>/numba/errors.py", line 595, in new_error_context
    yield
  File "<path>/numba/lowering.py", line 254, in lower_block
    self.lower_inst(inst)
  File "<path>/numba/lowering.py", line 358, in lower_inst
    return self.lower_setitem(inst.target, inst.index, inst.value, signature)
  File "<path>/numba/lowering.py", line 416, in lower_setitem
    impl = self.context.get_function('setitem', signature)
  File "<path>/numba/targets/base.py", line 537, in get_function
    return self.get_function(fn, sig, _firstcall=False)
  File "<path>/numba/targets/base.py", line 539, in get_function
    raise NotImplementedError("No definition for lowering %s%s" % (key, sig))
NotImplementedError: No definition for lowering setitem(array.flat(array(float64, 2d, C)), slice<a:b:c>, array.flat(array(float64, 2d, C))) -> none

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "foo2.py", line 10, in <module>
    flat(X, d)
  File "<path>/numba/dispatcher.py", line 368, in _compile_for_args
    raise e
  File "<path>/numba/dispatcher.py", line 325, in _compile_for_args
    return self.compile(tuple(argtypes))
  File "<path>/numba/dispatcher.py", line 653, in compile
    cres = self._compiler.compile(args, return_type)
  File "<path>/numba/dispatcher.py", line 83, in compile
    pipeline_class=self.pipeline_class)
  File "<path>/numba/compiler.py", line 881, in compile_extra
    return pipeline.compile_extra(func)
  File "<path>/numba/compiler.py", line 368, in compile_extra
    return self._compile_bytecode()
  File "<path>/numba/compiler.py", line 812, in _compile_bytecode
    return self._compile_core()
  File "<path>/numba/compiler.py", line 799, in _compile_core
    res = pm.run(self.status)
  File "<path>/numba/compiler.py", line 254, in run
    raise patched_exception
  File "<path>/numba/compiler.py", line 246, in run
    stage()
  File "<path>/numba/compiler.py", line 679, in stage_nopython_backend
    self._backend(lowerfn, objectmode=False)
  File "<path>/numba/compiler.py", line 629, in _backend
    lowered = lowerfn()
  File "<path>/numba/compiler.py", line 616, in backend_nopython_mode
    self.flags)
  File "<path>/numba/compiler.py", line 1000, in native_lowering_stage
    lower.lower()
  File "<path>/numba/lowering.py", line 173, in lower
    self.lower_normal_function(self.fndesc)
  File "<path>/numba/lowering.py", line 214, in lower_normal_function
    entry_block_tail = self.lower_function_body()
  File "<path>/numba/lowering.py", line 239, in lower_function_body
    self.lower_block(block)
  File "<path>/numba/lowering.py", line 254, in lower_block
    self.lower_inst(inst)
  File "<env>/lib/python3.6/contextlib.py", line 99, in __exit__
    self.gen.throw(type, value, traceback)
  File "<path>/numba/errors.py", line 603, in new_error_context
    six.reraise(type(newerr), newerr, tb)
  File "<path>/numba/six.py", line 658, in reraise
    raise value.with_traceback(tb)
  File "<path>/numba/errors.py", line 595, in new_error_context
    yield
  File "<path>/numba/lowering.py", line 254, in lower_block
    self.lower_inst(inst)
  File "<path>/numba/lowering.py", line 358, in lower_inst
    return self.lower_setitem(inst.target, inst.index, inst.value, signature)
  File "<path>/numba/lowering.py", line 416, in lower_setitem
    impl = self.context.get_function('setitem', signature)
  File "<path>/numba/targets/base.py", line 537, in get_function
    return self.get_function(fn, sig, _firstcall=False)
  File "<path>/numba/targets/base.py", line 539, in get_function
    raise NotImplementedError("No definition for lowering %s%s" % (key, sig))
numba.errors.LoweringError: Failed at nopython (nopython mode backend)
No definition for lowering setitem(array.flat(array(float64, 2d, C)), slice<a:b:c>, array.flat(array(float64, 2d, C))) -> none

File "foo2.py", line 6:
def flat(X, d):
    <source elided>
    N = X.shape[0]
    X.flat[:N**2:N+1] = d