`lookup_semiconcrete` returns `IRCode` regardless of setting of `optimize`

[timholy]

Clear in the code: https://github.com/JuliaDebug/Cthulhu.jl/blob/a6f7aabe27a780830e41d9d21bee3fa13133581b/src/Cthulhu.jl#L391-L399

I'm not (yet) sufficiently familiar with the internals to guess what we should do, but to me it seems either (1) it should not accept optimize as an argument, or (2) it should recompute src in a manner that respects the setting of optimize.

EDIT: but now I see Cthulhu maintains both src and codeinf, so maybe it's all OK. Feel free to close.