sroa: Mark dead setfields as EFFECT_FREE (#50373)
sroa tries to delete any `setfield!` call for allocations that it
knows it can remove. However, if it does not know that the type is
correct for the allocation, it may not be able to remove the
setfield!. If the type later gets improved (e.g. by irinterp),
the statement becomes eligible for removal, but it currently requires
another sroa pass to actually remove it.
Improve that situation my marking such a statement that is known-dead
as IR_FLAG_EFFECT_FREE, so if we later also prove it nothrow, it
(and the corresponding allocation) immediately become DCE-eligible.
