tree-optimization/104942: Retain sizetype conversions till the end

tree-optimization/104942: Retain sizetype conversions till the end

Retain the sizetype alloc_object_size to guarantee the assertion in
size_for_offset and to avoid adding a conversion there.  nop conversions
are eliminated at the end anyway in dynamic object size computation.

gcc/ChangeLog:

	PR tree-optimization/104942
	* tree-object-size.cc (alloc_object_size): Remove STRIP_NOPS.

gcc/testsuite/ChangeLog:

	PR tree-optimization/104942
	* gcc.dg/builtin-dynamic-object-size-0.c (alloc_func_long,
	test_builtin_malloc_long): New functions.
	(main): Use it.

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