[flang] Fix code generation for polymorphic deallocation

This patch fix issues found during call to deallocate.
fir.class can be handled the same way as fir.box in fir.store operation
code generation. In MutableBox::createNewFirBox, the fir.class is
also already a boxed entity.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D137079