[RISCV][TTI] Model the cost of extractelt when it cannot using vmv + vslide.

This patch implement the cost when the size of the vector need to split
into multiple groups and the index exceed single vector group.
Under this situation, we need the store the vector to stack and load the
target element.

After this patch, the cost of extract element will close to the
generated assembly.

Author: ElvisWang123

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2003,6 +2003,23 @@ InstructionCost RISCVTTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val,
     // TODO: should we count these special vsetvlis?
     BaseCost = Opcode == Instruction::InsertElement ? 3 : 4;
   }
+
+  // When the vector need to split into multiple register groups and the index
+  // exceed single vector resgister group, we need to extract the element via
+  // stack.
+  if (Opcode == Instruction::ExtractElement && LT.first > 1 &&
+      ((Index == -1U) || (Index > LT.second.getVectorMinNumElements() &&
+                          LT.second.isScalableVector()))) {
+    Type *ScalarType = Val->getScalarType();
+    Align VecAlign = DL.getPrefTypeAlign(Val);
+    Align SclAlign = DL.getPrefTypeAlign(ScalarType);
+    // Store all split vectors into stack and load the target element.
+    return LT.first *
+               getMemoryOpCost(Instruction::Store, Val, VecAlign, 0, CostKind) +
+           getMemoryOpCost(Instruction::Load, ScalarType, SclAlign, 0,
+                           CostKind);
+  }
+
   return BaseCost + SlideCost;
 }