[LAA] Use computeConstantDifference() (#103725)

Use computeConstantDifference() instead of casting getMinusSCEV() to
SCEVConstant. This can be much faster in some cases, because
computeConstantDifference() computes the result without creating new
SCEV expressions.

This improves LTO/ThinLTO compile-time for lencod by more than 10%.

I've verified that computeConstantDifference() does not produce worse
results than the previous code for anything in llvm-test-suite. This
required raising the iteration cutoff to 6. I ended up increasing it to
8 just to be on the safe side (for code outside llvm-test-suite), and
because this doesn't materially affect compile-time anyway (we'll almost
always bail out earlier).

Author: nikic

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -412,12 +412,10 @@ bool RuntimePointerChecking::needsChecking(
 /// Return nullptr in case we couldn't find an answer.
 static const SCEV *getMinFromExprs(const SCEV *I, const SCEV *J,
                                    ScalarEvolution *SE) {
-  const SCEV *Diff = SE->getMinusSCEV(J, I);
-  const SCEVConstant *C = dyn_cast<const SCEVConstant>(Diff);
-
-  if (!C)
+  std::optional<APInt> Diff = SE->computeConstantDifference(J, I);
+  if (!Diff)
     return nullptr;
-  return C->getValue()->isNegative() ? J : I;
+  return Diff->isNegative() ? J : I;
 }
 
 bool RuntimeCheckingPtrGroup::addPointer(
@@ -1599,11 +1597,11 @@ std::optional<int> llvm::getPointersDiff(Type *ElemTyA, Value *PtrA,
     // Otherwise compute the distance with SCEV between the base pointers.
     const SCEV *PtrSCEVA = SE.getSCEV(PtrA);
     const SCEV *PtrSCEVB = SE.getSCEV(PtrB);
-    const auto *Diff =
-        dyn_cast<SCEVConstant>(SE.getMinusSCEV(PtrSCEVB, PtrSCEVA));
+    std::optional<APInt> Diff =
+        SE.computeConstantDifference(PtrSCEVB, PtrSCEVA);
     if (!Diff)
       return std::nullopt;
-    Val = Diff->getAPInt().getSExtValue();
+    Val = Diff->getSExtValue();
   }
   int Size = DL.getTypeStoreSize(ElemTyA);
   int Dist = Val / Size;

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -11956,7 +11956,7 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
   APInt DiffMul(BW, 1);
   // Try various simplifications to reduce the difference to a constant. Limit
   // the number of allowed simplifications to keep compile-time low.
-  for (unsigned I = 0; I < 5; ++I) {
+  for (unsigned I = 0; I < 8; ++I) {
     if (More == Less)
       return Diff;