[llvm] [ADT] Update hash function of uint64_t for DenseMap (PR #95734)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

I think this makes sense on its own and we can improve the hash function without going all in on a better dense map implementation.

However, it would be good to have data on how this actually affects performance on real-world programs. For example, you could manually instrument dense map-ish containers and count the number of key operations (resize here?) when bootstrapping clang or compiling some other larger inputs.

https://github.com/llvm/llvm-project/pull/95734