Fix -fcompare-debug issue in purge_dead_edges [PR95080]

Fix -fcompare-debug issue in purge_dead_edges [PR95080]

The following testcase fails with -fcompare-debug, the bug used to be latent
since introduction of -fcompare-debug.
The loop at the start of purge_dead_edges behaves differently between -g0
and -g - if the last insn is a DEBUG_INSN, then it skips not just
DEBUG_INSNs but also NOTEs until it finds some other real insn (or bb head),
while with -g0 it will not skip any NOTEs, so if we have
real_insn
note
debug_insn // not present with -g0
then with -g it might remove useless REG_EH_REGION from real_insn, while
with -g0 it will not.

Yet another option would be not skipping NOTE_P in the loop; I couldn't find
in history rationale for why it is done.

2020-05-13  Jakub Jelinek  <jakub@redhat.com>

	PR debug/95080
	* cfgrtl.c (purge_dead_edges): Skip over debug and note insns even
	if the last insn is a note.

	* g++.dg/opt/pr95080.C: New test.