Fix debug info for SelectionDAG legalization of DAG nodes with two results.

This patch fixes the debug info handling for SelectionDAG legalization
of DAG nodes with two results. When an replaced SDNode has more than
one result, transferDbgValues was always copying the SDDbgValue from
the first result and attaching them to all members. In reality
SelectionDAG::ReplaceAllUsesWith() is given an array of SDNodes
(though the type signature doesn't make this obvious (cf. the call
site code in ReplaceNode()).

rdar://problem/44162227

Differential Revision: https://reviews.llvm.org/D52112

llvm-svn: 342264