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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

At this point, I think it would be safest to revert, reevaluate and test the known problematic cases before trying to reland this.

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