compiler/nir: Increment shader input count and mark as used when adding new gl_PointCoord
In case a new gl_PointCoord shader input is created, increment shader input count and set valid driver_location to the new input variable, otherwise the input gets aliased to input 0 and shows up in NIR_PRINT output as whatever shader input 0 is instead of gl_PointCoord. Also set the input as used, otherwise it might get removed.
Note that this is related to the following MR: !8618 (merged)