[SelectionDAG] Consolidate (t|T)ransferDbgValues methods, NFC (reapply)

TransferDbgValues (capital 'T') is wired into ReplaceAllUsesWith, and
transferDbgValues (lowercase 't') is used elsewhere (e.g in Legalize).

Both functions should be doing the exact same thing. This patch
consolidates the logic into one place.

This was reverted in r318455 because some newly introduced asserts,
which I thought were NFC, were firing. I filed PR35338. For now I've
weakened the asserts.

Testing: check-llvm, check-clang, and a stage2 Rel+Deb build of clang

Differential Revision: https://reviews.llvm.org/D40104

llvm-svn: 318498