This is a follow-up to
https://gcc.gnu.org/ml/gcc-patches/2019-11/msg00919.html (r278095).
Dominance info is deleted even if we don't perform jump threading.
Since the whole point of this pass is to perform jump threading (other
cleanups are not valuable at this point), skip it completely when
flag_thread_jumps is not set.

gcc/ChangeLog:

2019-11-14  Ilya Leoshkevich  <iii@linux.ibm.com>

	PR rtl-optimization/92430
	* cfgcleanup.c (pass_jump_after_combine::gate): New function.
	(pass_jump_after_combine::execute): Perform jump threading
	unconditionally.

From-SVN: r278254