Fix bug introduced by warning patch.
Fix bug introduced by warning patch.

Reverting the change in signedness of the `lane_id` variable fixes a
bug in optimized device code.

tl;dr:

It looks like the change in signedness of the `lane_id` introduced
a bug in a ReduceStep implementation, in which the expression

```
int offset = ...;
int last_lane = ...;
if (offset + lane_id <= last_lane) { /*... */ }
```

broke, because:

- `operator+(int, int)` returns `int`
- `operator+(int, uint32_t)` return `uint32_t`
- `last_lane` may be negative

My initial assumption that `last_lane` and `offset` would always
be positive was not correct.

In instances where `last_lane` is negative (which does happen
in the thrust.test.reduce TestReduceWithIndirectionDevice test),
this expression will always evaluate false, since an unsigned
value can't be <= a negative number.

So leave the ids as signed, since that seems necessary for this usecase.

NVlabs/CUB,NVlabs/cub,NVIDIA/cub