[llvm] [ADT] Avoid creating iterators in DenseMap::operator[] (NFC) (PR #155862)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

It appears this has resolved the issue, see: https://llvm-compile-time-tracker.com/compare.php?from=e2a55b8698e93de68e0080181844303cd820a3ae&to=f875a73af958bd1ed6c39849c7ae90371cd25ccb&stat=instructions:u

https://github.com/llvm/llvm-project/pull/155862