Bring fieldtype typefunc closer to the fieldtype builtin. Helps #7652

This fixes at least the reported problem and another one where the additional getfield inference smartness could create discrepancies between inferred and untyped code.
It is not a complete solution since we could infer more precise types for fields that are abstract, but I see that the same kind of simplification is done at some other places (typeof, ...) so it should be ok.