Reapply [GlobalOpt] Remove unreachable blocks before optimizing a function.

This commit reapplies r307215 now that we found out and fixed
the cause of the cfi test failure (in r307871).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307920 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed