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

[Reid Kleckner via llvm-commits]

rnk wrote:

The commit that introduced the Optional/std::optional 8a4ad8849f4898dd19e31b9dcede7ace3575d00d basically fixed this bug for complex SCEVs, but it didn't touch the value cache. Regardless, I think removing it is best if it doesn't regress compile time.

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