Allow user to overload cfg_simplify!

[MilesCranmer]

Allows users to overload the method Cthulhu.cfg_simplify!

(NOTE Cthulhu.cfg_simplify! != CC.cfg_simplify! so this isn't messing with internals of the compiler).

This is so that power users can just turn it off in their startup.jl and avoid this super annoying bug: #523 which seems like we are just waiting on Julia 1.11 for a fix.

Here's how to use it. In your startup.jl file:

using Cthulhu  # Regular import

# Rewrite buggy code:
import Cthulhu: cfg_simplify!
cfg_simplify!(ir) = ir

[aviatesk]

I think the right fix here would be to disable cfg_simplify! on v1.10 at all, or replace it with the proper round-trip. See #548.

[MilesCranmer]

SGTM, thanks!