[DebugInfo][LSR] Add more stringent checks on IV selection and salvage

attempts

Prevent the selection of IVs that have a SCEV containing an undef. Also
prevent salvaging attempts for values for which a SCEV could not be
created by ScalarEvolution and have only SCEVUknown.

Reviewed by: Orlando

Differential Revision: https://reviews.llvm.org/D111810