[flang] Load fir.ref<fir.class<T>> instead of creating a wrong box

When a subroutine has an entry statement, the non-used argument
will be a fir.alloca and result in a fir.ref<fir.class<T>> for
polymorphic entities. In createBox, just load the box instead of
creating a wrong box.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D146079