Infinite compile time for multibody quadrotor

[baggepinnen]

mtkcompile spends several minutes printing warnings like "Internal error: Variable xxx was marked as being in 0 ... but was actually zero". Compilation does not finish in 10 minutes.
https://github.com/JuliaComputing/Multibody.jl/blob/linear_scc_branch/docs/src/examples/quad.md

[AayushSabharwal]

On #4126 with the requisite dependency PRs, in a fresh REPL, mtkcompile finishes in a few seconds on my system. It results in a model with 73 unknowns. Around 30 warnings seem to be printed, which isn't enough to bottleneck simplification.