[flang] Do not try to rebox for assumed type

Just use conversion from assuemd type to assumed type
because a rebox needs more information that are not available
with assumed type.

Also use `isAssumedType` instead of `isBoxNone` since assumed
type can have sequence information.

Depends on D146207

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D146209