Improve LegacyPassManager API to correctly report modified status

When calling on-the-fly passes from the legacy pass manager, the modification
status is not reported, which is a problem in case we depend on an acutal
transformation pass, and not only analyse.

Update the Legacy PM API to optionally report the changed status, assert if a
change is detected but this change is lost.

Related to https://reviews.llvm.org/D80916

Differential Revision: https://reviews.llvm.org/D81236