intel/fs: Don't copy-propagate stride=0 sources into ddx/ddy
This can come up if, for instance, the shader does a derivative of a uniform or flat input. Ideally, NIR would use divergence analysis to get rid of the derivative in this case but it doesn't right now. This fixes a crash in F1 2017.
Cc: mesa-stable@lists.freedesktop.org Reported-by: Marcin Ślusarz marcin.slusarz@intel.com