[MachineOutliner] Fix label outlining regression introduced in D125072

Due to a change in the APIs used to determine what instructions can be outlined, the check for label outling was never hit. Instead, all labels were considered invisible, which is the opposite of the intended behavior and causes obscure crashes down the line. We now replicate the original behavior more closely, with explicit checks for known-good and known-bad instruction types.

Reviewed by: paquette

Differential Revision: https://reviews.llvm.org/D147178