[LAA] Split off code to compute stride from AddRec for reuse (NFC).

Refactors to code to expose the core logic from getPtrStride to compute
the stride for a given AddRec.

Split off from llvm#126971 as
suggested.

Author: fhahn

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -813,6 +813,48 @@ static bool hasComputableBounds(PredicatedScalarEvolution &PSE, Value *Ptr,
   return AR->isAffine();
 }
 
+/// Try to compute the stride for \p AR. Used by getPtrStride.
+static std::optional<int64_t>
+getStrideFromAddRec(const SCEVAddRecExpr *AR, const Loop *Lp, Type *AccessTy,
+                    Value *Ptr, PredicatedScalarEvolution &PSE) {
+  // The access function must stride over the innermost loop.
+  if (Lp != AR->getLoop()) {
+    LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
+                      << *Ptr << " SCEV: " << *AR << "\n");
+    return std::nullopt;
+  }
+
+  // Check the step is constant.
+  const SCEV *Step = AR->getStepRecurrence(*PSE.getSE());
+
+  // Calculate the pointer stride and check if it is constant.
+  const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
+  if (!C) {
+    LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr
+                      << " SCEV: " << *AR << "\n");
+    return std::nullopt;
+  }
+
+  const auto &DL = Lp->getHeader()->getDataLayout();
+  TypeSize AllocSize = DL.getTypeAllocSize(AccessTy);
+  int64_t Size = AllocSize.getFixedValue();
+  const APInt &APStepVal = C->getAPInt();
+
+  // Huge step value - give up.
+  if (APStepVal.getBitWidth() > 64)
+    return std::nullopt;
+
+  int64_t StepVal = APStepVal.getSExtValue();
+
+  // Strided access.
+  int64_t Stride = StepVal / Size;
+  int64_t Rem = StepVal % Size;
+  if (Rem)
+    return std::nullopt;
+
+  return Stride;
+}
+
 /// Check whether a pointer address cannot wrap.
 static bool isNoWrap(PredicatedScalarEvolution &PSE,
                      const DenseMap<Value *, const SCEV *> &Strides, Value *Ptr,
@@ -1458,42 +1500,9 @@ llvm::getPtrStride(PredicatedScalarEvolution &PSE, Type *AccessTy, Value *Ptr,
     return std::nullopt;
   }
 
-  // The access function must stride over the innermost loop.
-  if (Lp != AR->getLoop()) {
-    LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
-                      << *Ptr << " SCEV: " << *AR << "\n");
-    return std::nullopt;
-  }
-
-  // Check the step is constant.
-  const SCEV *Step = AR->getStepRecurrence(*PSE.getSE());
-
-  // Calculate the pointer stride and check if it is constant.
-  const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
-  if (!C) {
-    LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr
-                      << " SCEV: " << *AR << "\n");
-    return std::nullopt;
-  }
-
-  const auto &DL = Lp->getHeader()->getDataLayout();
-  TypeSize AllocSize = DL.getTypeAllocSize(AccessTy);
-  int64_t Size = AllocSize.getFixedValue();
-  const APInt &APStepVal = C->getAPInt();
-
-  // Huge step value - give up.
-  if (APStepVal.getBitWidth() > 64)
-    return std::nullopt;
-
-  int64_t StepVal = APStepVal.getSExtValue();
-
-  // Strided access.
-  int64_t Stride = StepVal / Size;
-  int64_t Rem = StepVal % Size;
-  if (Rem)
-    return std::nullopt;
-
-  if (!ShouldCheckWrap)
+  std::optional<int64_t> Stride =
+      getStrideFromAddRec(AR, Lp, AccessTy, Ptr, PSE);
+  if (!ShouldCheckWrap || !Stride)
     return Stride;
 
   // The address calculation must not wrap. Otherwise, a dependence could be