i965/fp: Emit discard jumps.

This should improve the performance of any shaders using the KIL
instruction.  I'm a bit surprised we missed this.

Unfortunately, I have not been able to measure any performance
improvements from this patch.  It does make ARB_fragment_program
behave similarly to GLSL code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_fs_fp.cpp

@@ -250,6 +250,9 @@ fs_visitor::emit_fragment_program_code()
                                     fs_reg(0.0f), BRW_CONDITIONAL_GE));
             cmp->predicate = BRW_PREDICATE_NORMAL;
             cmp->flag_subreg = 1;
+
+            if (brw->gen >= 6)
+               emit_discard_jump();
          }
          break;
       }