[llvm] Fix compiler errors in GVN.h for clang versions older than 15 (PR #107636)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM, this seems to fix my build on Apple Clang 12 (at least it seems to not fail for quite a while).

https://github.com/llvm/llvm-project/pull/107636