PM: Port DCE to the new pass manager

Also add a very basic test, since apparently there aren't any tests
for DCE whatsoever to add the new pass version to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267196 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed