add_subexpressions_for_field_reads is broken

With commit 59dce1c1 the add_subexpressions_for_field_reads simplification is broken, because the return statement for an early breakout does not follow the expected function signature. I guess an return ac with an extended test will fix the issue https://i10git.cs.fau.de/pycodegen/pystencils/-/blob/master/src/pystencils/simp/simplifications.py?#L192-193