df: Don't set bbs dirty because of debug insn moves [PR104459]

As mentioned in the PR, we get -fcompare-debug failure, which is caused by
cfg_layout_merge_blocks successfully merging two bbs where both bbs
contained just CODE_LABEL, NOTE_INSN_BASIC_BLOCK and in the -g case both
some debug insns at the end.  cfg_layout_merge_blocks calls
update_bb_for_insn_chain which for the post-label insns in the second block
(except for BARRIERs) calls df_insn_change_bb.  This function changes
the bb of the insns and for notes just punts, but for other insns calls
df_set_bb_dirty.  Now the problem is that because there were only debug
insns and notes in the second block, df_set_bb_dirty is called on both
only in the -g case and not with -g0.  df_set_bb_dirty these days
sets both the BB_MODIFIED flag and marks the bb as dirty, and the former
is what 6 spots in cfgcleanup.cc use in code-generation decisions,
in this case
          may_thread |= (target->flags & BB_MODIFIED) != 0;
in particular.  So, with -g may_thread is true while with -g0 it is not
and we diverge from that point onwards.
I've thought about introducing df_set_bb_dirty_nondebug that wouldn't
set BB_MODIFIED but would mark the bb dirty, but then I went through
history and found changes like:
https://gcc.gnu.org/legacy-ml/gcc-patches/2010-10/msg00059.html
so I've also tried just not calling df_set_bb_dirty for debug insns
at all and it passed x86_64-linux and i686-linux
--enable-checking=yes,rtl,extra,df bootstraps/regtests, so perhaps
that works too.
Now that I look at it again, if we don't need those from %d to %d messages
for debug insns in the dump files, another way to fix it would be just to
change the very first line in the hunk from
  if (!INSN_P (insn))
to
  if (!DEBUG_INSN_P (insn))
Though, df_set_bb_dirty_nondebug which will do everything but
set bb->flags |= BB_MODIFIED is yet another option I can test.
Perhaps even that PR42889 was solely about those 6 decisions in cfgcleanup
(at that point it used df_get_bb_dirty) and not about actually the
recomputation of some of the problems causing different code generations.

2022-02-11  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/104459
	* df-scan.cc (df_insn_change_bb): Don't call df_set_bb_dirty when
	moving DEBUG_INSNs between bbs.

	* gcc.dg/pr104459.c: New test.