[DebugInfo][LoopStrengthReduction] SCEV-based salvaging for LSR

This reapplies commit 76f3ffb2 that was
reverted due to buildbot failures.

- Update lit tests with REQUIRES condition.
- Abandon salvage attempt if SCEVUnknown::getValue() returns nullptr.

Differential Revision: https://reviews.llvm.org/D105207