calls: When bypassing emit_push_insn for 0 sized arg, emit at least anti_adjust_stack...

calls: When bypassing emit_push_insn for 0 sized arg, emit at least anti_adjust_stack for alignment pad if needed [PR104558]

The following testcase ICEs on x86_64 when asked to use the pre-GCC 8
ABI where zero sized arguments weren't ignored.
In GCC 7 the emit_push_insn calls in store_one_arg were unconditional,
it is true that they didn't actually push anything because it had zero
size, but because arg->locate.alignment_pad is 8 in this case,
emit_push_insn at the end performs
          if (alignment_pad && args_addr == 0)
            anti_adjust_stack (alignment_pad);
and an assert larger on is upset if we don't do it.
The following patch keeps the emit_push_insn conditional but calls
the anti_adjust_stack when needed by hand for the zero sized arguments.
For the new x86_64 ABI where zero sized arguments are ignored
arg->locate.alignment_pad is 0 in this case, so nothing changes
- we in that case really do ignore it.

There is another emit_push_insn call earlier in store_one_arg, also made
conditional on non-zero size by Marek in GCC 8, but that one is for
arguments with non-BLKmode and the only way those can be zero size is
if they are TYPE_EMPTY_P aka when they are completely ignored.  But
I believe arg->locate.alignment_pad should be 0 in that case, so IMHO
there is no need to do anything in the second spot.

2022-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/104558
	* calls.cc (store_one_arg): When not calling emit_push_insn
	because size_rtx is const0_rtx, call at least anti_adjust_stack
	on arg->locate.alignment_pad if !argblock and the alignment might
	be non-zero.

	* gcc.dg/pr104558.c: New test.