cp15: Add assertions to DC_InvalidateRange()

As mentioned in the commit changes: It's dangerous to invalidate a memory range. If the memory range isn't fully contained inside cache lines, invalidating it will also invalidate the variables around the range the caller wants to invalidate. This may cause unintended effects. I haven't applied this change yet because it will need more testing, but it will be merged over the next few days (it may even find issues in libnds...).