next: problem parsing if stmt

[havogt]

The following code fails, but looks legal

    def foo(a: bool, b: gtx.Field[[TDim], float64], c: gtx.Field[[TDim], float64]):
        res = b
        if a:
            return b
        else:
            res = c
        return res

with

gt4py.next.errors.exceptions.DSLError: Undeclared symbol 'resᐞ1'.
in

src/gt4py/next/ffront/foast_passes/type_deduction.py:354: DSLError