[SIL] Require `TransformIterator` support random access
`BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader()` was changed in
LLVM 7243607867393a2b8ccd477e95e6f62d00f3206f to use `llvm::size` rather
than the checking that `child_begin() + 1 == child_end()`. `llvm::size`
requires that `std::distance` be O(1) and hence `TransformIterator`
(`swift::SILBasicBlock::succblock_iterator` defined as the child
iterator type in the graph traits for `SILBasicBlock *`) support random
access.
The change to `llvm::size` seems rather pointless, so we could probably
also revert that if need be.
