[MachineOutliner] NFC: Hide LRU-related stuff behind helper functions

It's not particularly user-friendly to have to call `initLRU` everywhere. Also,
it wasn't particularly great that the LRU for registers used in a sequence was
also initialized by `initLRU`.

This patch hides this stuff behind some helper functions:

* `isAvailableAcrossAndOutOfSeq`
* `isAnyUnavailableAcrossOrOutOfSeq`
* `isAvailableInsideSeq`

This allows the user to avoid calling `initLRU` explicitly. Also, it allows
us to separate initializing the used-in-sequence LRU from the main LRU.

Since both ARM and AArch64 check LR liveness in `insertOutlinedCall`, this
refactor requires that we de-const the Candidate there.

Some other quality-of-code improvements:

* LRUs in outliner::Candidate now have more descriptive names
* Use `Register` instead of `unsigned` in some places
* Improve readability in some places by using ranges rather than `std::for_each`

This is a preparatory commit for a larger compile time related change for the
AArch64 outliner.

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