Refactor code from inlining and globalopt that checks whether a function definition...

Refactor code from inlining and globalopt that checks whether a function definition is unused, and enhance it so it can tell that functions which are only used by a blockaddress are in fact dead.  This probably doesn't happen much on most code, but the Linux kernel's _THIS_IP_ can trigger this issue with blockaddress.  (GlobalDCE can also handle the given tescase, but we only run that at -O3.)  Found while looking at PR11180.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142572 91177308-0d34-0410-b5e6-96231b3b80d8