[MachineOutliner] Make getOutliningType partially target-independent

[duck-37]

Summary

The motivation behind this change is to make fixing this (and other issues like this that may crop up in the future) easier.
This is _almost_ NFC, but 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 the case in RISCV, but other architectures still did hardcoded checks.
   • As an exception to this, CFI instructions are explicitly delegated to the target because RISCV 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, but just removing this check may trip someone up down the line.
3. isCFIIndex() and isFI() checks are also replaced with asserts, since these operands should not exist in any context at this stage in the pipeline.

[duck-37]

Comment clarification and typo fix.

[duck-37]

Rebase and add assertions for other operands that shouldn't exist at this point.

[pcwang-thead]

I'm not qualified for reviewing this patch, but it makes sense to me. Thanks for doing these changes. :-)

[pcwang-thead]

llvm/include/llvm/CodeGen/TargetInstrInfo.h

1937

We may need to change \p MI to \p MIT or modify param name.

llvm/lib/CodeGen/TargetInstrInfo.cpp

1458

"frame indices" could be excluded since we assert them now. :-)

[duck-37]

Rebase, update a few comments.

[duck-37]

Rebase.

[duck-37]

Rebase.