I've changed the semantics of MERGE_VALUES a bit. It's now allowed to live until...

I've changed the semantics of MERGE_VALUES a bit. It's now allowed to live until scheduling. It's deleted when the scheduler translate DAG nodes to machine instructions.

This is currently used by X86 to handle atomic_load_add when the output of the node is not used. I believe there is a better solution. But I find MERGE_VALUES useful for selecting multi-output node when the dead output can be selected as a IMPLICIT_DEF.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77583 91177308-0d34-0410-b5e6-96231b3b80d8