[Support] Do not use `llvm::size` in `getLoopPreheader` (#94540)

`BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader()` was changed in
72436078 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 that clients support
random access. Use `llvm::hasSingleElement` instead.