A split can use the same var name for inner and outer without error

The following code should throw an error on the call to split but does not. Instead it compiles to a garbage pipeline that sprays writes all over the place.

```
#include "Halide.h"

using namespace Halide;

int main(int argc, char **argv) {
    Var x;
    Func f;
    f(x) = 0;
    f.split(x, x, x, 16, TailStrategy::RoundUp);
    return 0;
}
```