[DebugInfo][LSR] Emit shorter expressions from scev-based salvaging

The scev-based salvaging for LSR can sometimes produce unnecessarily
verbose expressions. This patch adds logic to detect when the value to
be recovered and the induction variable differ by only a constant
offset. Then, the expression to derive the current iteration count can
be omitted from the dbg.value in favour of the offset.

Reviewed by: aprantl

Differential Revision: https://reviews.llvm.org/D109044