tree-optimization/104009: Conservative underflow estimate in object size

tree-optimization/104009: Conservative underflow estimate in object size

Restrict negative offset computation only to dynamic object sizes, where
size expressions are accurate and not a maximum/minimum estimate and in
cases where negative offsets definitely mean an underflow, e.g. in
MEM_REF of the whole object with negative ofset in addr_object_size.

This ends up missing some cases where __builtin_object_size could have
come up with more precise results, so tests have been adjusted to
reflect that.

gcc/ChangeLog:

	PR tree-optimization/104009
	* tree-object-size.c (compute_builtin_object_size): Bail out on
	negative offset.
	(plus_stmt_object_size): Return maximum of wholesize and minimum
	of 0 for negative offset.

gcc/testsuite/ChangeLog:

	PR tree-optimization/104009
	* gcc.dg/builtin-object-size-1.c (test10): New test.
	* gcc.dg/builtin-object-size-3.c (test10): Likewise.
	(test9): Expect zero size for negative offsets.
	* gcc.dg/builtin-object-size-4.c (test8): Likewise.
	* gcc.dg/builtin-object-size-5.c (test7): Drop test for
	__builtin_object_size.

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