Enable jump threading at -O1.

Enable jump threading at -O1.

My previous patch gating all jump threading by -fthread-jumps had the
side effect of turning off DOM jump threading at -O1.  This causes
numerous -Wuninitialized false positives.  This patch turns on jump
threading at -O1 to minimize the disruption.

gcc/ChangeLog:

	* cfgcleanup.c (pass_jump::execute): Check
	flag_expensive_optimizations.
	(pass_jump_after_combine::gate): Same.
	* doc/invoke.texi (-fthread-jumps): Enable for -O1.
	* opts.c (default_options_table): Enable -fthread-jumps at -O1.
	* tree-ssa-threadupdate.c
	(fwd_jt_path_registry::remove_jump_threads_including): Bail unless
	flag_thread_jumps.

gcc/testsuite/ChangeLog:

	* gcc.dg/auto-init-uninit-1.c: Adjust.
	* gcc.dg/auto-init-uninit-15.c: Same.
	* gcc.dg/guality/example.c: Same.
	* gcc.dg/loop-8.c: Same.
	* gcc.dg/strlenopt-40.c: Same.
	* gcc.dg/tree-ssa/pr18133-2.c: Same.
	* gcc.dg/tree-ssa/pr18134.c: Same.
	* gcc.dg/uninit-1.c: Same.
	* gcc.dg/uninit-pr44547.c: Same.
	* gcc.dg/uninit-pr59970.c: Same.