Make this pass substantially stronger by having it delete dead return values

as well as arguments.  Now it can delete arguments and return values which are
only passed into other arguments or are returned, if they are dead.  This causes
it to delete several hundred extra args/retvals from the C++ hello world program,
shrinking it by about 2K.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9398 91177308-0d34-0410-b5e6-96231b3b80d8