Pass manager: add support for termination of pass list

Pass manager: add support for termination of pass list

	* cgraphunit.c (cgraph_node::expand_thunk): Call
	allocate_struct_function before init_function_start.
	(cgraph_node::expand): Use push_cfun and pop_cfun.
	* config/i386/i386.c (ix86_code_end): Call
	allocate_struct_function before init_function_start.
	* config/rs6000/rs6000.c (rs6000_code_end): Likewise.
	* function.c (init_function_start): Move preamble to all
	callers.
	* passes.c (do_per_function_toporder): Use push_cfun and pop_cfun.
	(execute_one_pass): Handle newly added TODO_discard_function.
	(execute_pass_list_1): Terminate if cfun equals to NULL.
	(execute_pass_list): Do not push and pop cfun, expect that
	cfun is set.
	* tree-pass.h (TODO_discard_function): Define.

From-SVN: r229764