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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

Max RSS:

- Vanilla
  - 1,312,988 KB
- #218156 
  - 1,331,496 KB
- #218188
  - 1,327,496 KB
- #218190
  - 1,352,932 KB

The Max RSS here is from compiling `SLPVectorizer.cpp`.  These numbers are preliminary.  I have not tried to stabilize these numbers (disabling ASLR, THP, etc).


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