`lp.assignment_to_subst`: Should it support 1-long axes as a special case?

```python
knl = lp.make_kernel(
'{[i, j]: 0<=i,j<10}',
"""
<> tmp[i, 0] = 2 * i
out[j] = tmp[j, 0]
""")

knl = lp.assignment_to_subst(knl, 'tmp')
```

fails with:

```
File "/home/square/projects/ceesd/loopy/loopy/transform/subst.py", line 431, in assignment_to_subst
raise LoopyError("In defining instruction '%s': "
loopy.diagnostic.LoopyError: In defining instruction 'insn': asignee index '0' is not a plain variable. Perhaps use loopy.affine_map_inames() to perform substitution.
```

Should we support such trivial definitions in `assignment_to_subst` or is it the user's problem to avoid having such definitions in the first place?

Start with the reproducer in the issue and inspect loopy/transform/subst.py around assignment_to_subst, especially the error at line 431. Compare the behavior with the suggested affine_map_inames() path and determine whether one-long axes should be supported; done means the expected behavior and corresponding validation are established.

Written by the indexing model from the issue text.