[Local] Sink salvageDI's early exit into helper functions, NFC

salvageDebugInfo() performs a check that allows it to exit early without
doing a DenseMap lookup. It's a bit neater and marginally more useful to
sink this early exit into the findDbg{Addr,Users,Values} helpers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335642 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed