[DebugInfo][NFCI] Split salvageDebugInfo into helper functions

Some use cases are appearing where salvaging is needed that does not
correspond to an instruction being deleted -- for example an instruction
being sunk, or a Value not being available in a block being isel'd.

Enable more fine grained control over how salavging occurs by splitting
the logic into helper functions, separating things that are specific to
working on DbgVariableIntrinsics from those specific to interpreting IR
and building DIExpressions.

Differential Revision: https://reviews.llvm.org/D57696

llvm-svn: 353156