Assignments to Vertical Field cause failed Assertion in PassSetStageLocationType

[mroethlin]

If the left hand side of an assignment is a FieldAccessExpression, the set location type pass tries to retrieve the horizontal dimension of the left hand side here without bothering to check whether the field is vertical. This means stencils like:

@stencil
def vert2vert_assign(
    inF: Field[K], outF: Field[K]):
    with levels_downward as k:
        outF = inF

fail during lowering.

Currently, this has no IIR representation. We should probably wait and see if this has an actual use case.