i386: Fix setup of incoming varargs for (...) functions which return large aggregates [PR114175]

The c23-stdarg-6.c testcase I've added recently apparently works fine with
-O0 but aborts with -O1 and higher on x86_64-linux.
The problem is in setup of incoming varargs.

Like function.cc before r14-9249 even ix86_setup_incoming_varargs assumes
that TYPE_NO_NAMED_ARGS_STDARG_P don't have any named arguments and there
is nothing to advance, but that is not the case for (...) functions
returning by hidden reference which have one such artificial argument.
If the setup_incoming_varargs hook is called from the
  if (TYPE_NO_NAMED_ARGS_STDARG_P (TREE_TYPE (fndecl))
      && fnargs.is_empty ())
    {
      struct assign_parm_data_one data = {};
      assign_parms_setup_varargs (&all, &data, false);
    }
spot, i.e. where there is no hidden return argument passed, arg.type
is always NULL, while when it is called in the
      if (cfun->stdarg && !DECL_CHAIN (parm))
        assign_parms_setup_varargs (&all, &data, false);
spot, even when it is TYPE_NO_NAMED_ARGS_STDARG_P arg.type will be non-NULL.
The tree-stdarg.cc pass in f in c23-stdarg-6.cc at -O1 or higher determines
that va_arg is used on integral types at most twice (loads 2 words),
and because ix86_setup_incoming_varargs doesn't advance, the code saves
just the %rdi and %rsi registers to the save area.  But that isn't correct,
it should save %rsi and %rdx because %rdi is the hidden return argument.
With -O0 tree-stdarg.cc doesn't attempt to optimize and we save all the
registers, so it works fine in that case.

Now, I think we'll need the same fix also on
aarch64, alpha, arc, csky, ia64, loongarch, mips, mmix, nios2, riscv, visium
which have pretty much the similarly looking snippet in their hooks
changed by the r13-3549 commit.
Then arm, epiphany, fr30, frv, ft32, m32r, mcore, nds32, rs6000, sh
have different changes but most likely need something similar too.
I don't have access to most of those, could test aarch64 and rs6000 I guess.

2024-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR target/114175
	* config/i386/i386.cc (ix86_setup_incoming_varargs): Only skip
	ix86_function_arg_advance for TYPE_NO_NAMED_ARGS_STDARG_P functions
	if arg.type is NULL.

	* gcc.dg/c23-stdarg-7.c: New test.
	* gcc.dg/c23-stdarg-8.c: New test.