[flang] fix procedure fir.box_addr identification in boxed-procedure (#79290)

The pass was mistakenly identifying a fir.box_addr on a
fir.box/fir.class of a derived type with procedure pointer components as
being a fir.box_addr on a procedure.

Simply check if the input type is a fir.box_proc or function type (if
input already rewritten) and insert convert only in this case.

This caused "invalid fir.convert" internal error.