Make flag_thread_jumps a gate of pass_jump_after_combine

Make flag_thread_jumps a gate of pass_jump_after_combine

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