{DebugInfo][LSR] Don't cache dbg.value that are already undef

The SCEV-based salvaging method caches dbg.value information pre-LSR so
that salvaging may be attempted post-LSR. If the dbg.value are already
undef pre-LSR then a salvage attempt would be fruitless, so avoid
caching them.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D107448