[flang] Fix hlfir.as_expr codegen for polymorphic entities (#80824)

https://github.com/llvm/llvm-project/pull/80683 revealed that
hlfir.as_expr was propagating the temporary buffer for polymorphic
values as an allocatable while codegen later expects to be working with
fir.box/fir.class but not fir.ref<box/class> when processing the
operations using the hlfir.as_expr result.

Dereference the temporary allocatable as soon as it is created.