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

This reapplies commit 76f3ffb 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

Author: Chris Jackson

llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h

@@ -83,6 +83,9 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
   /// InsertedValues/InsertedPostIncValues.
   SmallPtrSet<Value *, 16> ReusedValues;
 
+  // The induction variables generated.
+  SmallVector<WeakVH, 2> InsertedIVs;
+
   /// A memoization of the "relevant" loop for a given SCEV.
   DenseMap<const SCEV *, const Loop *> RelevantLoops;
 
@@ -199,9 +202,11 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
     InsertedPostIncValues.clear();
     ReusedValues.clear();
     ChainedPhis.clear();
+    InsertedIVs.clear();
   }
 
   ScalarEvolution *getSE() { return &SE; }
+  const SmallVectorImpl<WeakVH> &getInsertedIVs() const { return InsertedIVs; }
 
   /// Return a vector containing all instructions inserted during expansion.
   SmallVector<Instruction *, 32> getAllInsertedInstructions() const {