tree-object-size: Always set computed bit for bdos [PR113012]

tree-object-size: Always set computed bit for bdos [PR113012]

It is always safe to set the computed bit for dynamic object sizes at
the end of collect_object_sizes_for because even in case of a dependency
loop encountered in nested calls, we have an SSA temporary to actually
finish the object size expression.  The reexamine pass for dynamic
object sizes is only for propagation of unknowns and gimplification of
the size expressions, not for loop resolution as in the case of static
object sizes.

gcc/ChangeLog:

	PR tree-optimization/113012
	* tree-object-size.cc (compute_builtin_object_size): Expand
	comment for dynamic object sizes.
	(collect_object_sizes_for): Always set COMPUTED bitmap for
	dynamic object sizes.

gcc/testsuite/ChangeLog:

	PR tree-optimization/113012
	* gcc.dg/ubsan/pr113012.c: New test case.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>
(cherry picked from commit 576c1fc4401a9dae9757ac2e4fa37d05e130fa3d)