[IRCE][NFC] Extract 'IV vs Limit' parsing to a separate method

[aleksandr.popov]

Summary

Next step of the preparatory refactoring for the upcoming support of new
new range check form to parse.

Previous one: https://reviews.llvm.org/D154158

This change isolates logic of 'IV vs Limit' range check parsing to
simplify adding parsers for new range checks forms.

llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

static bool parseRangeCheckICmp(Loop *L, ICmpInst *ICI, ScalarEvolution &SE,
                                   const SCEVAddRecExpr *&Index,
                                   const SCEV *&End);
 
   static void
   extractRangeChecksFromCond(Loop *L, ScalarEvolution &SE, Use &ConditionUse,
                              SmallVectorImpl<InductiveRangeCheck> &Checks,
                              SmallPtrSetImpl<Value *> &Visited);
 
+  static bool parseIvAgaisntLimit(Loop *L, Value *LHS, Value *RHS,
+                                  ICmpInst::Predicate Pred, ScalarEvolution &SE,
+                                  const SCEVAddRecExpr *&Index,
+                                  const SCEV *&End);
+
 public:
   const SCEV *getBegin() const { return Begin; }
   const SCEV *getStep() const { return Step; }
   const SCEV *getEnd() const { return End; }
 
   void print(raw_ostream &OS) const {
     OS << "InductiveRangeCheck:\n";
     OS << "  Begin: ";

 bool InductiveRangeCheck::parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
                                               ScalarEvolution &SE,
                                               const SCEVAddRecExpr *&Index,
                                               const SCEV *&End) {
   auto IsLoopInvariant = [&SE, L](Value *V) {
     return SE.isLoopInvariant(SE.getSCEV(V), L);
   };
 
-  auto SIntMaxSCEV = [&](Type *T) {
-    unsigned BitWidth = cast<IntegerType>(T)->getBitWidth();
-    return SE.getConstant(APInt::getSignedMaxValue(BitWidth));
-  };
-
   ICmpInst::Predicate Pred = ICI->getPredicate();
   Value *LHS = ICI->getOperand(0);
   Value *RHS = ICI->getOperand(1);
 
   // Canonicalize to the `Index Pred Invariant` comparison
   if (IsLoopInvariant(LHS)) {
     std::swap(LHS, RHS);
     Pred = CmpInst::getSwappedPredicate(Pred);
   } else if (!IsLoopInvariant(RHS))
     // Both LHS and RHS are loop variant
     return false;
 
+  if (parseIvAgaisntLimit(L, LHS, RHS, Pred, SE, Index, End))
+    return true;
+
+  return false;
+}
+
+// Try to parse range check in the form of "IV vs Limit"
+bool InductiveRangeCheck::parseIvAgaisntLimit(Loop *L, Value *LHS, Value *RHS,
+                                              ICmpInst::Predicate Pred,
+                                              ScalarEvolution &SE,
+                                              const SCEVAddRecExpr *&Index,
+                                              const SCEV *&End) {
+
+  auto SIntMaxSCEV = [&](Type *T) {
+    unsigned BitWidth = cast<IntegerType>(T)->getBitWidth();
+    return SE.getConstant(APInt::getSignedMaxValue(BitWidth));
+  };
+
   const auto *AddRec = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(LHS));
   if (!AddRec)
     return false;
 
   // We strengthen "0 <= I" to "0 <= I < INT_SMAX" and "I < L" to "0 <= I < L".
   // We can potentially do much better here.
   // If we want to adjust upper bound for the unsigned range check as we do it
   // for signed one, we will need to pick Unsigned max