[RISCV][TTI]Fix the cost estimation for long select shuffle.

The code was broken completely. Need to iterate over the whole mask and
process the submasks correctly, check if they form full indentity and
adjust indices correctly.

Fixes #106126

Author: alexey-bataev

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -447,21 +447,30 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
         auto *SubVecTy = FixedVectorType::get(Tp->getElementType(), SubVF);
 
         InstructionCost Cost = 0;
-        for (unsigned I = 0; I < NumRegs; ++I) {
+        for (unsigned I = 0, NumSrcRegs = divideCeil(Mask.size(), SubVF);
+             I < NumSrcRegs; ++I) {
           bool IsSingleVector = true;
           SmallVector<int> SubMask(SubVF, PoisonMaskElem);
-          transform(Mask.slice(I * SubVF,
-                               I == NumRegs - 1 ? Mask.size() % SubVF : SubVF),
-                    SubMask.begin(), [&](int I) {
-                      bool SingleSubVector = I / VF == 0;
-                      IsSingleVector &= SingleSubVector;
-                      return (SingleSubVector ? 0 : 1) * SubVF + I % VF;
-                    });
+          transform(
+              Mask.slice(I * SubVF,
+                         I == NumSrcRegs - 1 ? Mask.size() % SubVF : SubVF),
+              SubMask.begin(), [&](int I) -> int {
+                if (I == PoisonMaskElem)
+                  return PoisonMaskElem;
+                bool SingleSubVector = I / VF == 0;
+                IsSingleVector &= SingleSubVector;
+                return (SingleSubVector ? 0 : 1) * SubVF + (I % VF) % SubVF;
+              });
+          if (all_of(enumerate(SubMask), [](auto &&P) {
+                return P.value() == PoisonMaskElem ||
+                       static_cast<unsigned>(P.value()) == P.index();
+              }))
+            continue;
           Cost += getShuffleCost(IsSingleVector ? TTI::SK_PermuteSingleSrc
                                                 : TTI::SK_PermuteTwoSrc,
                                  SubVecTy, SubMask, CostKind, 0, nullptr);
-          return Cost;
         }
+        return Cost;
       }
       break;
     }