[llvm] [ADT] Speed up FoldingSet with Algorithm R (PR #218188)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@MaskRay Did you have a chance to look at Max RSS on a reasonably large source files?  `FoldingSet` shows up at the peak heap usage, for example, while compiling `SLPVectorizer.cpp`.  IIUC, your #218190 places 32-bit hash value at two places -- in the table and also in the `FoldingSetNode`.

https://github.com/llvm/llvm-project/pull/218188