[llvm] [SCEV] Remove EqCacheSCEV (PR #133186)

[Reid Kleckner via llvm-commits]

rnk wrote:

+1 to everything Arthur said. Your fix would fix the assertion as well. Returning std::optional as an addition might be a nice improvement, but IIRC we don't even use the 3-way return value, we boil it all down to a strict-less-than comparator, so having a 4-way return value (UNKNOWN, LT, EQ, GT) seems like extra complexity. UNKNOWN and EQ are equivalent for sorting purposes here, the SCEVs are equal for the purpose of prefix comparison.

https://github.com/llvm/llvm-project/pull/133186