[turbofan] Induction variable decrement bound analysis.

src/compiler/loop-variable-optimizer.h

Index: src/compiler/loop-variable-optimizer.h
diff --git a/src/compiler/loop-variable-optimizer.h b/src/compiler/loop-variable-optimizer.h
index c7e82dc6f5bd8e85bc87012cfa9a04a225130f64..a5c1ad448d89433f0e0328c152bc6571e692cebd 100644
--- a/src/compiler/loop-variable-optimizer.h
+++ b/src/compiler/loop-variable-optimizer.h
@@ -23,6 +23,7 @@ class InductionVariable : public ZoneObject {
   Node* init_value() const { return init_value_; }
 
   enum ConstraintKind { kStrict, kNonStrict };
+  enum ArithmeticType { kAddition, kSubtraction };
   struct Bound {
     Bound(Node* bound, ConstraintKind kind) : bound(bound), kind(kind) {}
 
@@ -33,17 +34,20 @@ class InductionVariable : public ZoneObject {
   const ZoneVector<Bound>& lower_bounds() { return lower_bounds_; }
   const ZoneVector<Bound>& upper_bounds() { return upper_bounds_; }
 
+  ArithmeticType Type() { return arithmeticType_; }
+
  private:
   friend class LoopVariableOptimizer;
 
   InductionVariable(Node* phi, Node* arith, Node* increment, Node* init_value,
-                    Zone* zone)
+                    Zone* zone, ArithmeticType arithmeticType)
       : phi_(phi),
         arith_(arith),
         increment_(increment),
         init_value_(init_value),
         lower_bounds_(zone),
-        upper_bounds_(zone) {}
+        upper_bounds_(zone),
+        arithmeticType_(arithmeticType) {}
 
   void AddUpperBound(Node* bound, ConstraintKind kind);
   void AddLowerBound(Node* bound, ConstraintKind kind);
@@ -54,6 +58,7 @@ class InductionVariable : public ZoneObject {
   Node* init_value_;
   ZoneVector<Bound> lower_bounds_;
   ZoneVector<Bound> upper_bounds_;
+  ArithmeticType arithmeticType_;
 };
 
 class LoopVariableOptimizer {