In #97711 the single-store optimization was disabled for the case
where the value is potentially poison, as this may produce incorrect
results for loads of uninitialized memory.

However, this resulted in compile-time regressions. Address these
by still allowing the single-store optimization to occur in cases
where the store dominates the load, as we know that such a load
will always read initialized memory.

(cherry picked from commit daaea128bb84f8ed7b9de36aa3a51f33b775c05a)