[LSR] Early exit for RateFormula when it is already losing. NFC

[eopXD]

Summary

This patch does not effect any behavior of the current code.

The codebase implicitly implies that Cost::RateFormula is only called
when the Cost is not in losing status, or else there may be possible
to trigger the assertion of Cost::isValid.

The intention here is to prevent mis-use where future development
allow Cost that is already loser to call Cost::RateFormula - Early
exit when Cost is already losing.

[Meinersbur]

Can you already trigger the isValid with the current code or is intended for you future changes? If the former, is it possible to add a test case? If the latter, could you highlight that in the summery (maybe even add "NFC" in the title)?

[eopXD]

In D125670#3542657, @Meinersbur wrote:

Can you already trigger the isValid with the current code or is intended for you future changes? If the former, is it possible to add a test case? If the latter, could you highlight that in the summery (maybe even add "NFC" in the title)?

The current code does not trigger isValid. Yes, this patch should be an NFC.

[eopXD]

Gentle ping, thank you.

[Meinersbur]

LGTM. Thanks you.

[eopXD]

Update to main.

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

void Cost::RatePrimaryRegister(const Formula &F, const SCEV *Reg,
   }
 }
 
 void Cost::RateFormula(const Formula &F,
                        SmallPtrSetImpl<const SCEV *> &Regs,
                        const DenseSet<const SCEV *> &VisitedRegs,
                        const LSRUse &LU,
                        SmallPtrSetImpl<const SCEV *> *LoserRegs) {
+  if (isLoser())
+    return;
   assert(F.isCanonical(*L) && "Cost is accurate only for canonical formula");
   // Tally up the registers.
   unsigned PrevAddRecCost = C.AddRecCost;
   unsigned PrevNumRegs = C.NumRegs;
   unsigned PrevNumBaseAdds = C.NumBaseAdds;
   if (const SCEV *ScaledReg = F.ScaledReg) {
     if (VisitedRegs.count(ScaledReg)) {
       Lose();