Debug info: Don't crash in SelectionDAGISel when a vreg that is being
pointed to by a dbg_value belonging to a function argument is eliminated
during instruction selection.
rdar://problem/15094721.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192011 91177308-0d34-0410-b5e6-96231b3b80d8