lower-bitint: Remove single label _BitInt switches [PR113737]

The following testcase ICEs, because group_case_labels_stmt optimizes
  switch (a.0_7) <default: <L6> [50.00%], case 0: <L7> [50.00%], case 2: <L7> [50.00%]>
where L7 block starts with __builtin_unreachable (); to
  switch (a.0_7) <default: <L6> [50.00%]>
and single label GIMPLE_SWITCH is something the switch expansion refuses to
lower:
  if (gimple_switch_num_labels (m_switch) == 1
      || range_check_type (index_type) == NULL_TREE)
    return false;
(range_check_type never returns NULL for BITINT_TYPE), but the gimple
lowering pass relies on all large/huge _BitInt switches to be lowered
by that pass.

The following patch just removes those after making the single successor
edge EDGE_FALLTHRU.  I've done it even if !optimize just in case in case
we'd end up with single case label from earlier passes.

2024-02-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113737
	* gimple-lower-bitint.cc (gimple_lower_bitint): If GIMPLE_SWITCH
	has just a single label, remove it and make single successor edge
	EDGE_FALLTHRU.

	* gcc.dg/bitint-84.c: New test.