Cleanup TransferDbgValues

[DAG] Check debug values for invalidation before transferring and mark
old debug values invalid when transferring to another SDValue.

This fixes PR28613.

Reviewers: jyknight, hans, dblaikie, echristo

Subscribers: yaron.keren, ismail, llvm-commits

Differential Revision: https://reviews.llvm.org/D22858

llvm-svn: 277135