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

This patch extends salvaging of debuginfo in the Loop Strength Reduction
(LSR) pass by translating Scalar Evaluations (SCEV) into DIExpressions.
The method is as follows:
- Cache dbg.value intrinsics that are salvageable.
- Obtain a loop Induction Variable (IV) from ScalarExpressionExpander or
  the loop header.
- Translate the IV SCEV into an expression that recovers the current
  loop iteration count. Combine this with the dbg.value's location
  op SCEV to create a DIExpression that salvages the value.

Review by: jmorse

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