DebugInfo: Simplify dead variable collection slightly.

constructSubprogramDIE was already called for every subprogram in every
CU when the module was started - there's no need to call it again at
module finalization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209372 91177308-0d34-0410-b5e6-96231b3b80d8