The following testcase ICEs, because during the cfg cleanup, we see:
  switch (i$e_11) <default: <L12> [33.33%], case -3: <lab2> [33.33%], case 0: <L10> [33.33%], case 2: <lab2> [33.33%]>
...
lab2:
  __builtin_unreachable ();
where lab2 is FORCED_LABEL.  The way it works, we go through the case labels
and when we reach the first one that points to gimple_seq_unreachable*
basic block, we remove the edge (if any) from the switch bb to the bb
containing the label and bbs reachable only through that edge we've just
removed.  Once we do that, we must throw away all other cases that use
the same label (or some other labels from the same bb we've removed the edge
to and the bb).  To avoid quadratic behavior, this is not done by walking
all remaining cases immediately before removing, but only when processing
them later.
For normal labels this works, fine, if the label is in a deleted bb, it will
have NULL label_to_block and we handle that case, or, if the unreachable bb
has some other edge to it, only the edge will be removed and not the bb,
and again, find_edge will not find the edge and we only remove the case.
And if a label would be to some other block, that other block wouldn't have
been removed earlier because there would be still an edge from the switch
block.
Now, FORCED_LABEL (and I think DECL_NONLOCAL too) break this, because
those labels aren't removed, but instead moved to some surrounding basic
block.  So, when we later process those, when their gimple_seq_unreachable*
basic block is removed, label_to_block will return some unrelated block
(in the testcase the switch bb), so we decide to keep the case which doesn't
seem to be unreachable, but we don't really have an edge from the switch
block to the block the label got moved to.

I thought first about punting in gimple_seq_unreachable* on
FORCED_LABEL/DECL_NONLOCAL labels, but that might penalize even code that
doesn't care, so this instead just makes sure that for
FORCED_LABEL/DECL_NONLOCAL labels that are being removed (and thus moved
randomly) we remember in a hash_set the fact that those labels should be
treated as removed for the purpose of the optimization, and later on
handle those labels that way.

2020-07-02  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/95857
	* tree-cfg.c (group_case_labels_stmt): When removing an unreachable
	base_bb, remember all forced and non-local labels on it and later
	treat those as if they have NULL label_to_block.  Formatting fix.
	Fix a comment typo.

	* gcc.dg/pr95857.c: New test.

(cherry picked from commit 00f24f56732861d09a9716fa5b6b8a96c2289143)