[GlobalDCE, Misc] Don't remove functions referenced by ifuncs

We forgot to consider the target of ifuncs when considering if a
function was alive or dead.

N.B. Also update a few auxiliary tools like bugpoint and
verify-uselistorder.

This fixes PR27593.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268468 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed