[ADCE] Fix incorrect reporting of CFG changes

This patch fixes 2 related bugs in ADCE:
- `performDeadCodeElimination` does not report changes if it did ONLY
  CFG changes (affects both old and new pass managers);
- When control flow removal is enabled, new pass manager does not
  drop CFG analyses.

Both can lead to incorrect loop info after ADCE that does only CFG changes.

Differential Revision: https://reviews.llvm.org/D78103
Reviewed By: Denis Antrushin