tree-object-size: Robustify alloc_size attribute handling [PR113013]

The following testcase ICEs because we aren't careful enough with
alloc_size attribute.  We do check that such an argument exists
(although wouldn't handle correctly functions with more than INT_MAX
arguments), but didn't check that it is scalar integer, the ICE is
trying to fold_convert a structure to sizetype.

Given that the attribute can also appear on non-prototyped functions
where the arguments aren't known, I don't see how the FE could diagnose
that and because we already handle the case where argument doesn't exist,
I think we should also verify the argument is scalar integer convertible
to sizetype.  Furthermore, given this is not just in diagnostics but
used for code generation, I think it is better to punt on arguments with
larger precision then sizetype, the upper bits are then truncated.

The patch also fixes some formatting issues and avoids duplication of the
fold_convert, plus removes unnecessary check for if (arg1 >= 0), that is
always the case after if (arg1 < 0) return ...;

2023-12-18  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113013
	* tree-object-size.cc (alloc_object_size): Return size_unknown if
	corresponding argument(s) don't have integral type or have integral
	type with higher precision than sizetype.  Don't check arg1 >= 0
	uselessly.  Compare argument indexes against gimple_call_num_args
	in unsigned type rather than int.  Formatting fixes.

	* gcc.dg/pr113013.c: New test.

(cherry picked from commit 5347263b)