i965: check inst->predicate when clearing flag_live at dead code eliminate

Detected by Matt Turner while reviewing commit
a59359ecd2

Reviewed-by: Matt Turner <mattst88@gmail.com>

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

@@ -105,7 +105,7 @@ fs_visitor::dead_code_eliminate()
             }
          }
 
-         if (inst->writes_flag()) {
+         if (inst->writes_flag() && !inst->predicate) {
             BITSET_CLEAR(flag_live, inst->flag_subreg);
          }
 

src/mesa/drivers/dri/i965/brw_vec4_dead_code_eliminate.cpp

@@ -145,7 +145,7 @@ vec4_visitor::dead_code_eliminate()
             }
          }
 
-         if (inst->writes_flag()) {
+         if (inst->writes_flag() && !inst->predicate) {
             for (unsigned c = 0; c < 4; c++)
                BITSET_CLEAR(flag_live, c);
          }