PR27938: Don't remove valid DebugLoc in Scalarizer

Added checks to make sure the Scalarizer::transferMetadata() don't
remove valid debug locations from instructions. This is important as
the verifier pass require that e.g. inlinable callsites have a valid
debug location.

https://llvm.org/bugs/show_bug.cgi?id=27938

Patch by Karl-Johan Karlsson

Reviewers: dblaikie

Differential Revision: http://reviews.llvm.org/D20807

llvm-svn: 272884