Free dominance info at the beginning of pass_jump_after_combine

Free dominance info at the beginning of pass_jump_after_combine

try_forward_edges does not update dominance info, and merge_blocks
relies on it being up-to-date.  In PR92430 stale dominance info makes
merge_blocks produce a loop in the dominator tree, which in turn makes
delete_basic_block loop forever.

Fix by freeing dominance info at the beginning of cleanup_cfg.

Also, 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:

2019-11-15  Ilya Leoshkevich  <iii@linux.ibm.com>

	Backport from mainline
	PR rtl-optimization/92430
	* cfgcleanup.c (pass_jump_after_combine::gate): New function.
	(pass_jump_after_combine::execute): Free
	dominance info at the beginning.

gcc/testsuite/ChangeLog:

2019-11-15  Ilya Leoshkevich  <iii@linux.ibm.com>

	Backport from mainline
	PR rtl-optimization/92430
	* gcc.dg/pr92430.c: New test (from Arseny Solokha).

From-SVN: r278291