[RISCV][TTI] Discount slide cost if ri.vinsert/ri.vextract are available (#142036)

If we have the ri.vinsert/vextract instructions from xrivosvisni, we can
do an element insert or extract without needing a vslide or a vector
temporary register. Adjust the TTI cost to reflect this.

Author: preames

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2297,9 +2297,12 @@ InstructionCost RISCVTTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val,
       Index = Index % M1Max;
     }
 
-    // We could extract/insert the first element without vslidedown/vslideup.
     if (Index == 0)
+      // We can extract/insert the first element without vslidedown/vslideup.
       SlideCost = 0;
+    else if (ST->hasVendorXRivosVisni() && isUInt<5>(Index) &&
+             Val->getScalarType()->isIntegerTy())
+      SlideCost = 0; // With ri.vinsert/ri.vextract there is no slide needed
     else if (Opcode == Instruction::InsertElement)
       SlideCost = 1; // With a constant index, we do not need to use addi.
   }