[llvm] [ADT] Refactor DenseMap::insert, try_emplace, and operator[] (NFC) (PR #155204)

[Nikita Popov via llvm-commits]

nikic wrote:

This causes a large compile-time regression, especially when building with clang: https://llvm-compile-time-tracker.com/compare.php?from=790a132b8535e28d118ba3c9f5e02dd7853bbac4&to=2e896274bd4e61891824fce35f7e0552b2f4be4b&stat=instructions:u (+0.5%)

https://github.com/llvm/llvm-project/pull/155204