i965: Validate the IR tree after doing our custom optimization passes.

This wouldn't catch the last failure fixed in them, because we don't
validate assignments well (due to the fact that we've got a pretty
glaring inconsistency in how we handle assignment writemasking), but
it could catch other failure we may produce.

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

@@ -144,6 +144,8 @@ brw_link_shader(GLcontext *ctx, struct gl_shader_program *prog)
 	    progress = do_common_optimization(shader->ir, true) || progress;
 	 } while (progress);
 
+	 validate_ir_tree(shader->ir);
+
 	 reparent_ir(shader->ir, shader->ir);
 	 talloc_free(mem_ctx);
       }