Add `deletableInstructions()` and `reverseDeletableInstructions()` in SILBasicBlock.
It allows deleting instructions while iterating over all instructions of the block.
This is a replacement for `InstructionDeleter::updatingRange()`.
It's a simpler implementation than the existing `UpdatingListIterator` and `UpdatingInstructionIteratorRegistry`, because it just needs to keep the prev/next pointers for "deleted" instructions instead of the iterator-registration machinery.
It's also safer, because it doesn't require to delete instructions via a specific instance of an InstructionDeleter (which can be missed easily).
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