[flang][hlfir] Fixed hlfir.assign codegen for polymorphic LHS.

The RHS cannot be casted to the LHS type, when LHS is polymorphic.
With this change we will use the RHS type for emboxing with special
hadling for i1 type.

I created https://github.com/llvm/llvm-project/issues/62419 for the
AllocaOp generated during HLFIRtoFir conversion.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D149392