[mlir][LLVMIR] Use the correct way to determine if it's a scalable vector

One of the ShuffleVectorOp::build functions checks if the incoming
vector operands is scalable vector by casting its type to
mlir::VectorType first. However, in some cases the operand is not
necessarily mlir::VectorType (e.g. it might be a LLVMVectorType).

This patch fixes this issue by using the dedicated
`LLVM::isScalableVectorType` function to determine if the incoming
vector is scalable vector or not.

Differential Revision: https://reviews.llvm.org/D125818