df: Don't abuse bb->aux (PR94148, PR94042)

df: Don't abuse bb->aux (PR94148, PR94042)

The df dataflow solvers use the aux field in the basic_block struct,
although that is reserved for any use by passes.  And not only that,
it is required that you set all such fields to NULL before calling
the solvers, or you quietly get wrong results.

This changes the solvers to use a local array for last_change_age
instead, just like it already had a local array for last_visit_age.

	PR rtl-optimization/94148
	PR rtl-optimization/94042
	* df-core.c (BB_LAST_CHANGE_AGE): Delete.
	(df_worklist_propagate_forward): New parameter last_change_age, use
	that instead of bb->aux.
	(df_worklist_propagate_backward): Ditto.
	(df_worklist_dataflow_doublequeue): Use a local array last_change_age.