SelectionDAG: Remove arbitrary and bug-prone complexity from SDLoc

The implementation of SDLoc has an extra layer of indirection here for
no particular reason, and was leading to problems where we were
dereferencing pointers to SDNodes that had already been deleted so
that we could get at the DebugLoc for a new SDNode. This is one of the
errors that came up often in PR26808.

Instead, we can just track the DebugLoc and IROrder directly. This
makes the code both easier to understand and more correct. It's also
basically NFC other than fixing a large number of places where we were
reading the memory of deleted SDNodes.

llvm-svn: 264470