[IndVarSimplify] Fix Modified status for removal of overflow intrinsics

When removing an overflow intrinsic the Changed status in SimplifyIndvar
was not set, leading to the IndVarSimplify pass returning an incorrect
status.

This was caught using the check introduced by D80916.

As pointed out in the code review, a similar bug may exist for
eliminateTrunc().

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D85971