[NFC][SCEV] SCEVExpander::isHighCostExpansionHelper(): check that we processed expression first

[lebedev.ri]

Summary

As far as i can tell this is still NFC.
Initially in rL146438 it was added at the top of the function,
later rL238507 dethroned it, and rL244474 did it again.

I'm not sure if we have already checked the cost of this expansion, we should be doing that again.

[lebedev.ri]

Rebased, NFC.

[mkazantsev]

There is no possible situation when it matters, but fine. Making this check first sounds reasonable. LGTM.

[lebedev.ri]

Rebased, NFC.

[lebedev.ri]

Rebased, NFC.

llvm/lib/Analysis/ScalarEvolutionExpander.cpp

SCEVExpander::getRelatedExistingExpansion(const SCEV *S, const Instruction *At,
 
   // Can not find suitable value.
   return None;
 }
 
 bool SCEVExpander::isHighCostExpansionHelper(
     const SCEV *S, Loop *L, const Instruction *At, int &BudgetRemaining,
     const TargetTransformInfo *TTI, SmallPtrSetImpl<const SCEV *> &Processed) {
+  // Was the cost of expansion of this expression already accounted for?
+  if (!Processed.insert(S).second)
+    return false; // We have already accounted for this expression.
+
   // If we can find an existing value for this scev available at the point "At"
   // then consider the expression cheap.
   if (At && getRelatedExistingExpansion(S, At, L))
     return false;
 
   // Zero/One operand expressions
   switch (S->getSCEVType()) {
   case scUnknown:
   case scConstant:
     return false;
   case scTruncate:
     return isHighCostExpansionHelper(cast<SCEVTruncateExpr>(S)->getOperand(), L,
                                      At, BudgetRemaining, TTI, Processed);
   case scZeroExtend:
     return isHighCostExpansionHelper(cast<SCEVZeroExtendExpr>(S)->getOperand(),
                                      L, At, BudgetRemaining, TTI, Processed);
   case scSignExtend:
     return isHighCostExpansionHelper(cast<SCEVSignExtendExpr>(S)->getOperand(),
                                      L, At, BudgetRemaining, TTI, Processed);
   }
 
-  if (!Processed.insert(S).second)
-    return false;
 
   if (auto *UDivExpr = dyn_cast<SCEVUDivExpr>(S)) {
     // If the divisor is a power of two and the SCEV type fits in a native
     // integer (and the LHS not expensive), consider the division cheap
     // irrespective of whether it occurs in the user code since it can be
     // lowered into a right shift.
     if (auto *SC = dyn_cast<SCEVConstant>(UDivExpr->getRHS()))
       if (SC->getAPInt().isPowerOf2()) {