[MachineOutliner] Make getOutliningType partially target-independent

The motivation behind this patch is to unify some of the outliner logic across architectures. This looks nicer in general and makes fixing [issues like this](https://reviews.llvm.org/D124707#3483805) easier.
There are some notable changes here:
    1. `isMetaInstruction()` is used directly instead of checking for specific meta-instructions like `IMPLICIT_DEF` or `KILL`. This was already done in the RISC-V implementation, but other architectures still did hardcoded checks.
        - As an exception to this, CFI instructions are explicitly delegated to the target because RISC-V has different handling for those.

    2. `isTargetIndex()` checks are replaced with an assert; none of the architectures supported actually use `MO_TargetIndex` at this point in time.

    3. `isCFIIndex()` and `isFI()` checks are also replaced with asserts, since these operands should not exist in [any context](https://reviews.llvm.org/D122635#3447214) at this stage in the pipeline.

Reviewed by: paquette

Differential Revision: https://reviews.llvm.org/D125072