intel: Don't propagate conditional modifiers if a UD source is negated

This fixes a bug uncovered by my NIR integer division by constant
optimization series.

Fixes: 19f9cb72 "i965/fs: Add pass to propagate conditional..."
Fixes: 627f94b7 "i965/vec4: adding vec4_cmod_propagation..."
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 4ba445e0)