Rewrite the DeadArgumentElimination pass, to use a more explicit tracking of

dependencies between return values and/or arguments. Also make the handling of
arguments and return values the same.

The pass now looks properly inside returned structs, but only at the first                                                                                     
level (ie, not inside nested structs).

Also add a testcase for testing various variations of (multiple) dead rerturn
values.

llvm-svn: 52459