Add `Opt_ConstantFolding` to turn on constant folding (#20500)
Finish what we started in !6999 (closed): remove optLevel completely from DynFlags, and only use it
while processing CLI options. The Core-to-Core optimization pipeline always has the same shape now, with individual passes included or not based on individual optimization flags. One new optimization flag -fconstant-folding is needed to fully restore the previous -O1 behaviour.