[ADCE] Preserve MemorySSA if only debug instructions are removed

As we've seen in
 https://github.com/llvm/llvm-project/issues/58285
throwing away MemorySSA when only debug instructions are removed can lead
to different code when debug info is present or not present.

This is the second attempt at fixing the problem. The first one was
 https://reviews.llvm.org/D145051
which was reverted due to a 15% compile time regression for tramp3d-v4
in NewPM-ReleaseLTO-g.