[Debuginfo][LSR] Add salvaging variadic dbg.value intrinsics [2/2]

Second of two patches to extend SCEV-based salvaging to dbg.value
intrinsics that have multiple location ops pre-LSR. This second patch
adds the core implementation.

Reviewers: @StephenTozer, @djtodoro

Differential Revision: https://reviews.llvm.org/D120169