[flang] Do not propagate type desc when box type is not polymorphic

When the rhs is non-polymorphic the type descriptor should not
be propagated. An error in the EmboxOp verifier was raised in that case.
This patch propagate the type descriptor only if the result type of the
EmboxOp operation is polymorphic.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D138442