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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> is that on a old version of clang as well?

Yes, this was on Apple Clang 12, which should correspond to llvm.org Clang 10-11.

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