[Debuginfo][LSR] Add salvaging variadic dbg.value intrinsics [1/2] [NFC]

First of two patches that extends SCEV-based salvaging to enable
salvaging of dbg.value instrinsics that have multiple locations ops
before the Loop Strength Reduction pass.

The existing single-op SCEV-based salvaging can generate variadic
dbg.value intrinsics in order to salvage a dbg.value that has a single
location op. If a dbg.value has multiple location ops before LSR, and
LSR optimises away one or more of the location operands, then currently
no salvaging will be attempted.

Salvaging can now be added, but first this patch cleans up consistency
in both the code and comments, and applies some refactoring to make
application of the new salvaging implementation more straightforward.

- Use SCEVDbgValueBuilder for both types of recovery expressions:
  IV-offset based and iteration count based.
- Combine the functions that write the final DIExpression.
- Move some static functions into member functions.

Reviewers: @Orlando

Differential Revision: https://reviews.llvm.org/D120168