tree-optimization/103961: Never compute offset for -1 size

tree-optimization/103961: Never compute offset for -1 size

Never try to compute size for offset when the object size is -1, which
is either unknown maximum or uninitialized minimum irrespective of the
osi->pass number.

gcc/ChangeLog:

	PR tree-optimization/103961
	* tree-object-size.c (plus_stmt_object_size): Always avoid
	computing offset for -1 size.

gcc/testsuite/ChangeLog:

	PR tree-optimization/103961
	* gcc.dg/pr103961.c: New test case.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>
Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>