tree-inline: Fix up __builtin_va_arg_pack handling [PR100898]

tree-inline: Fix up __builtin_va_arg_pack handling [PR100898]

The following testcase ICEs, because gimple_call_arg_ptr (..., 0)
asserts that there is at least one argument, while we were using
it even if we didn't copy anything just to get a pointer from/to which
the zero arguments should be copied.

Fixed by guarding the memcpy calls.  Also, the code was calling
gimple_call_num_args too many times - 5 times instead of 2, so the patch
adds two temporaries for those.

2021-06-07  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/100898
	* tree-inline.c (copy_bb): Only use gimple_call_arg_ptr if memcpy
	should copy any arguments.  Don't call gimple_call_num_args
	on id->call_stmt or call_stmt more than once.

	* g++.dg/ext/va-arg-pack-3.C: New test.