Implement salavageDebugInfo functionality for SelectionDAG.

Similar to how llvm::salvagDebugInfo hooks into InstCombine, this adds
a hook that can be invoked before an SDNode that is associated with an
SDDbgValue is erased to capture the effect of the deleted node in a
DIExpression.

The motivating example is an SDDebugValue attached to an ADD operation
that gets folded into a LOAD+OFFSET operation.

rdar://problem/32121503

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316525 91177308-0d34-0410-b5e6-96231b3b80d8