[DebugInfo][DAG] Refactor dbg.value lowering into its own method

This is a pure copy-and-paste job, moving the logic for lowering dbg.value
intrinsics to SDDbgValues into its own function. This is ahead of adding some
more users of this logic.

Differential Revision: https://reviews.llvm.org/D57697


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353950 91177308-0d34-0410-b5e6-96231b3b80d8