passes: Enable split4 with selective scheduling 2 [PR98439]

passes: Enable split4 with selective scheduling 2 [PR98439]

As mentioned in the PR, we have 5 split passes (+ splitting during final).
split1 is before RA and is unconditional,
split2 is after RA and is gated on optimize > 0,
split3 is before sched2 and is gated on
  defined(INSN_SCHEDULING) && optimize > 0 && flag_schedule_insns_after_reload
split4 is before regstack and is gated on
  HAVE_ATTR_length && defined (STACK_REGS) && !gate (split3)
split5 is before shorten_branches and is gated on
  HAVE_ATTR_length && !defined (STACK_REGS)
and the splitting during final works only when !HAVE_ATTR_length.
STACK_REGS is a macro enabled only on i386/x86_64.

The problem with the following testcase is that split3 before sched2
is the last splitting pass for the target/command line options set,
but selective scheduling unlike normal scheduling can create new
instructions that need to be split, which means we ICE during final as
there are insns that require splitting but nothing split them.

This patch fixes it by doing split4 also when -fselective-scheduling2
is enabled on x86 and split3 has been run.  As that option isn't on
by default, it should slow down compilation only for those that enable
that option.

2021-02-13  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/98439
	* recog.c (pass_split_before_regstack::gate): Enable even when
	pass_split_before_sched2 is enabled if -fselective-scheduling2 is
	on.

	* gcc.target/i386/pr98439.c: New test.