tree-object-size: use size_for_offset in more cases

When wholesize != size, there is a reasonable opportunity for static
object sizes also to be computed using size_for_offset, so use that.

gcc/ChangeLog:

	* tree-object-size.cc (plus_stmt_object_size): Call
	SIZE_FOR_OFFSET for some negative offset cases.

gcc/testsuite/ChangeLog:

	* gcc.dg/builtin-object-size-3.c (test9): Adjust test.
	* gcc.dg/builtin-object-size-4.c (test8): Likewise.

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