intel/fs: Use regs_written() in spilling cost heuristic for improved accuracy.

intel/fs: Use regs_written() in spilling cost heuristic for improved accuracy.

This is what we use later on to compute the number of registers that
will actually get spilled to memory, so it's more likely to match
reality than the current open-coded approximation.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>