[llvm] [llvm] Use llvm::append_range (NFC) (PR #136066)

[Nikita Popov via llvm-commits]

nikic wrote:

Strictly speaking SmallVector::insert() should probably be requiring a forward iterator, not an input iterator, for it's current representation. (Though PostOrderIterator is also a forward iterator, but maybe we could not make it one.)

https://github.com/llvm/llvm-project/pull/136066