intel/fs: Don't copy-propagate stride=0 sources into ddx/ddy

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>
Tested-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6564>