[llvm] [RISCV][CostModel] Updates reduction and shuffle cost (PR #77342)

[Shih-Po Hung via llvm-commits]

================
@@ -482,11 +481,10 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
       //   vmsne.vi  v0, v8, 0
 
       return LT.first *
-             (TLI->getLMULCost(LT.second) + // FIXME: this should be 1 for andi
----------------
arcbbb wrote:

For SK_Broadcast, my understanding is that the index should always be the first element of the source vector. 
I was based on this
```
      if (Shuffle->isZeroEltSplat())
        return TargetTTI->getShuffleCost(TTI::SK_Broadcast, VecTy,
                                         Shuffle->getShuffleMask(), CostKind, 0,
                                         nullptr, Operands);
```
Could we clarify this?"

https://github.com/llvm/llvm-project/pull/77342