Recommit r52459, rewriting of the dead argument elimination pass.

This is a fixed version that no longer uses multimap::equal_range, which
resulted in a pointer invalidation problem.

Also, DAE::InspectedFunctions was not really necessary, so it got removed.

Lastly, this version no longer applies the extra arg hack on functions who did
not have any arguments to start with.

llvm-svn: 52532