[SCEV] Split computeExitLimitFromICmp into two versions [NFC]

This is in advance of a following change which needs to the non-icmp API.

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1713,6 +1713,17 @@ private:
                                      bool IsSubExpr,
                                      bool AllowPredicates = false);
 
+  /// Variant of previous which takes the components representing an ICmp
+  /// as opposed to the ICmpInst itself.  Note that the prior version can
+  /// return more precise results in some cases and is preferred when caller
+  /// has a materialized ICmp.
+  ExitLimit computeExitLimitFromICmp(const Loop *L, ICmpInst::Predicate Pred,
+                                     const SCEV *LHS, const SCEV *RHS,
+                                     bool ExitIfTrue,
+                                     bool IsSubExpr,
+                                     bool AllowPredicates = false);
+
+
   /// Compute the number of times the backedge of the specified loop will
   /// execute if its exit condition were a switch with a single exiting case
   /// to ExitingBB.

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -8203,6 +8203,28 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
   const SCEV *LHS = getSCEV(ExitCond->getOperand(0));
   const SCEV *RHS = getSCEV(ExitCond->getOperand(1));
 
+  ExitLimit EL = computeExitLimitFromICmp(L, Pred, LHS, RHS, ExitIfTrue,
+                                          ControlsExit, AllowPredicates);
+  if (EL.hasAnyInfo()) return EL;
+
+  auto *ExhaustiveCount =
+      computeExitCountExhaustively(L, ExitCond, ExitIfTrue);
+
+  if (!isa<SCEVCouldNotCompute>(ExhaustiveCount))
+    return ExhaustiveCount;
+
+  return computeShiftCompareExitLimit(ExitCond->getOperand(0),
+                                      ExitCond->getOperand(1), L, OriginalPred);
+}
+ScalarEvolution::ExitLimit
+ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
+                                          ICmpInst::Predicate Pred,
+                                          const SCEV *LHS, const SCEV *RHS,
+                                          bool ExitIfTrue,
+                                          bool ControlsExit,
+                                          bool AllowPredicates) {
+
+
   // Try to evaluate any dependencies out of the loop.
   LHS = getSCEVAtScope(LHS, L);
   RHS = getSCEVAtScope(RHS, L);
@@ -8312,14 +8334,7 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
     break;
   }
 
-  auto *ExhaustiveCount =
-      computeExitCountExhaustively(L, ExitCond, ExitIfTrue);
-
-  if (!isa<SCEVCouldNotCompute>(ExhaustiveCount))
-    return ExhaustiveCount;
-
-  return computeShiftCompareExitLimit(ExitCond->getOperand(0),
-                                      ExitCond->getOperand(1), L, OriginalPred);
+  return getCouldNotCompute();
 }
 
 ScalarEvolution::ExitLimit