[SCEV] Do not cache comparison result upon reached max depth as "equivalence". PR48725

We use `EquivalenceClasses` to cache the notion that two SCEVs are equivalent,
so save time in situation when `A` is equivalent to `B` and `B` is equivalent to `C`,
making check "if `A` is equivalent to `C`?" cheaper.

We also return `0` in the comparator when we reach max analysis depth to save
compile time. After doing this, we also cache them as being equivalent.

Now, imagine the following situation:
- `A` is proved equivalent to `B`;
- `C` is proved equivalent to `D`;
- Comparison of `A` against `D` is proved non-zero;
- Comparison of `B` against `C` reaches max depth (and gets cached as equivalence).

Now, before the invocation of compare(`B`, `C`), `A` and `D` belonged
to different equivalence classes, and their comparison returned non-zero.
After the the invocation of compare(`B`, `C`), equivalence classes get merged
and `A`, `B`, `C` and `D` all fall into the same equivalence class. So the comparator
will change its behavior for couple `A` and `D`, with weird consequences following it.
This comparator is finally used in `std::stable_sort`, and this behavior change
makes it crash (looks like it's causing a memory corruption).

Solution: this patch changes `CompareSCEVComplexity` to return `None`
when the max depth is reached. So in this case, we do not cache these SCEVs
(and their parents in the tree) as being equivalent.

Differential Revision: https://reviews.llvm.org/D94654
Reviewed By: lebedev.ri