[llvm][RISCV] Do not assume V extension on seeing vector type. (#166994)

We have a private extension which also uses the vector type in the
frontend. Our platform does not have the V extension, so it triggered
assertion failures from within getLMULCost().

It feels reasonable to check for V extension before assuming LMUL
exists.

Author: wecing

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2140,7 +2140,8 @@ InstructionCost RISCVTTIImpl::getMemoryOpCost(unsigned Opcode, Type *Src,
   // Assume memory ops cost scale with the number of vector registers
   // possible accessed by the instruction.  Note that BasicTTI already
   // handles the LT.first term for us.
-  if (LT.second.isVector() && CostKind != TTI::TCK_CodeSize)
+  if (ST->hasVInstructions() && LT.second.isVector() &&
+      CostKind != TTI::TCK_CodeSize)
     BaseCost *= TLI->getLMULCost(LT.second);
   return Cost + BaseCost;
 }