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

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

I believe this is only a partial fix because if we limit ourselves to three characters we can still end up in a situation like

```
abc
abcd
abce
abcd
```

and we don't end up deduplicating the `abcd`s. However, removing the depth limit may have compile time consequences, whereas the usefulness of the cache I think is unknown (discussed in https://reviews.llvm.org/D26389). this PR is an improvement on its own

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