[LoongArch] Override TargetLowering::isShuffleMaskLegal

 By default, `isShuffleMaskLegal` always returns true, which can result
 in the expansion of `BUILD_VECTOR` into a `VECTOR_SHUFFLE` node in
 certain situations. Subsequently, the `VECTOR_SHUFFLE` node is expanded
 again into a `BUILD_VECTOR`, leading to an infinite loop.
 To address this, we always return false, allowing the expansion of
 `BUILD_VECTOR` through the stack.