Add 64-bit hash_combine (#3041)
Add 64-bit hash_combine (#3041)

Currently only the low 32-bits of a hash are guaranteed to be shuffled before combining with the other hash, so this PR also adds a 64-bit variant of hash_combine, including a comment on where the constants are coming from.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen