fwprop: fix REG_DEAD notes rendered incorrect

[Eric Botcazou]

> fwprop may propagate a SET_SRC that contains the last use of a REG to a
> later point, but it will leave the REG_DEAD note in place, even though
> it is no longer correct.  I noticed this while investigating PR50826:
> the initialization of the internal_arg_pointer has a REG_DEAD note for
> %r29, but even after it is fwpropped, the note remains there.  This
> patch fixes this bug.

This isn't necessarily a bug, i.e. passes aren't required to maintain REG_DEAD 
and REG_UNUSED notes up-to-date (another example is PR rtl-opt/48773).  Instead 
passes consuming these notes have to invoke df_note_add_problem on entry.

-- 
Eric Botcazou