Restructure JIT to have more extension points and more logical contro…

…l flow (JuliaLang#50650)

Rather than forking the optimize and compile layers into individual
layers per optlevel, each optimize and compile layer will switch on the
requested optlevel of the module, which reduces the complexity of
tracing a module's path through the JIT. In addition, this lets us move
some of the `addModule` code to happen post-optimization, which makes
the optimization pipeline not see literal pointers except those
generated by codegen.