[LoopInfo] Replace raw loop-container accessors with semantic operations (#210653)

getBlocksVector, mutable getBlocksSet, getSubLoopsVector, and
getTopLevelLoopsVector expose LoopInfo's internal containers, so every
caller open-codes loop-nest surgery, keeps the block vector and set in
sync by hand, and freezes the representation. Replace them with two
LoopInfoBase operations:

* removeBlocksIf(LoopT &, Pred): drop matching blocks from a loop.
* takeChildrenIf(LoopT *Parent, Pred): detach and return matching
  children, clearing their parent; a null Parent means the top level.

Both preserve the relative order of what remains. Mutation inside
LoopInfoBase also lets the analysis repair derived state once per edit
and finish a multi-step edit before any query observes it, which an
accessor returning a container cannot.

For the same reason, move hasNoExitBlocks, getExitEdges, and
getUniqueLatchExitBlock onto LoopInfoBase.

Two behavior changes: SimpleLoopUnswitch's deleteDeadBlocksFromLoop runs
its per-child deletion callbacks while the forest is still consistent,
before detaching and destroying dead children; FixIrreducible's
reconnectChildLoops keeps children in their original order rather than
std::partition's.

Aided by Claude Fable 5

GitOrigin-RevId: 9a49fc244be7400e550b2c014e6d370b586dc854