Follow-up to D105207, only salvage affine SCEVs to avoid a crash

SCEVToIterCountExpr only expects to be fed affine expressions, but
DbgRewriteSalvageableDVIs is feeding it non-affine induction variables.
Following this up with an obvious fix, will add test coverage too if
this avoids D105207 being reverted.

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -6162,6 +6162,9 @@ DbgRewriteSalvageableDVIs(llvm::Loop *L, ScalarEvolution &SE,
  bool Changed = false;
  if (const SCEVAddRecExpr *IVAddRec =
  dyn_cast<SCEVAddRecExpr>(SCEVInductionVar)) {
+ if (!IVAddRec->isAffine())
+ return false;
+
  SCEVDbgValueBuilder IterCountExpr;
  IterCountExpr.pushValue(LSRInductionVar);
  if (!IterCountExpr.SCEVToIterCountExpr(*IVAddRec, SE))