Change `ModulePass::skipModule` to take const Module reference.
Additionally, make `OptPassGate::shouldRunPass` const as well as for
most implementations it's a const query. For `OptBisect`, make
`LastBisectNum` mutable so it could be updated in `shouldRunPass`.
Additional minor cleanup: Change all StringRef arguments to simple
StringRef (no const or reference), change `OptBisect::Disabled` to
constexpr.