tree-optimization/104941: Actually assign the conversion result

Assign the result of fold_convert to offset.  Also make the useless
conversion check lighter since the two way check is not needed here.

gcc/ChangeLog:

	PR tree-optimization/104941
	* tree-object-size.cc (size_for_offset): Make useless conversion
	check lighter and assign result of fold_convert to OFFSET.

gcc/testsuite/ChangeLog:

	PR tree-optimization/104941
	* gcc.dg/builtin-dynamic-object-size-0.c (S1, S2): New structs.
	(test_alloc_nested_structs, g): New functions.
	(main): Call test_alloc_nested_structs.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>