ac/nir: Sanitize location_frac for local variables.

If they were promoted from inputs/outputs, they could have a
non-zero value left over, which messed with our store handling.

Fixes: 06f05040 "radv: Link shaders."
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 67e09c8b)