Debug info: Factor the retrieving of the DIVariable from a MachineInstr
into a function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205973 91177308-0d34-0410-b5e6-96231b3b80d8
Follow-up to r205973: change the return type to const MDNode*.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205974 91177308-0d34-0410-b5e6-96231b3b80d8