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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52459 91177308-0d34-0410-b5e6-96231b3b80d8