tree-optimization/103456 - Record only successes from object_sizes_set

tree-optimization/103456 - Record only successes from object_sizes_set

Avoid overwriting osi->changed if object_sizes_set does not update the
size, so that a previous success in the same pass is not overwritten.
This fixes the bootstrap-ubsan build config, which was failing due to
incorrect object size.

gcc/ChangeLog:

	PR tree-optimization/103456
	* tree-object-size.c (merge_object_sizes): Update osi->changed
	only if object_sizes_set succeeded.

gcc/testsuite/ChangeLog:

	PR tree-optimization/103456
	* gcc.dg/ubsan/pr103456.c: New test.

Co-authored-by: Martin Liška <mliska@suse.cz>
Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>