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

[Reid Kleckner via llvm-commits]

rnk wrote:

Is this a partial fix? I guess the question is, is SCEV deduplication a correctness requirement? Is the depth cutoff sound?

This code is effectively trying to do a bucket sort, like sorting strings by the first three characters:
```
abc
abb
aba
abd
abcf
abce
abcd

--> 3 char depth sort -->

aba
abb
abc
abcf. # not strictly sorted, but ordered w.r.t others
abce # not strictly sorted
abcd # not strictly sorted
abd
```


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