[llvm] [NFC] Use stable_hash_combine instead of hash_combine (PR #105619)

[Fangrui Song via llvm-commits]

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

Perhaps worth a follow-up to change `stable_hash_combine` to 64-bit xxh3. The current FNV1 has bad performance

(I'm starting a 3-week vacation this Friday and will have limited availability.)

https://github.com/llvm/llvm-project/pull/105619