[SCEV] Help getLoopInvariantExitCondDuringFirstIterations deal with complex...

[SCEV] Help getLoopInvariantExitCondDuringFirstIterations deal with complex `umin` exit counts. PR59615

Recent improvements in symbolic exit count computation revealed some problems with
SCEV's ability to find invariant predicate during first iterations. Ultimately it is based on its
ability to prove some facts for value on the last iteration. This last value, when it includes
`umin` as part of exit count, isn't always simplified enough. The motivating example is following:

https://github.com/llvm/llvm-project/issues/59615

Could not prove:
```
        Pred = 36, LHS = (-1 + (-1 * (2147483645 umin (-1 + %var)<nsw>))<nsw> + %var), RHS = %var
        FoundPred = 36, FoundLHS = {1,+,1}<nuw><nsw><%bb3>, FoundRHS = %var
```
Can prove:
```
        Pred = 36, LHS = (-1 + (-1 * (-1 + %var)<nsw>)<nsw> + %var), RHS = %var
        FoundPred = 36, FoundLHS = {1,+,1}<nuw><nsw><%bb3>, FoundRHS = %var
```

Here ` (2147483645 umin (-1 + %var)<nsw>)` is exit count composed of two parts from
two different exits: `2147483645 ` and `(-1 + %var)<nsw>`. When it was only one (latter)
analyzeable exit, for it everything was easily provable. Unfortunately, in general case `umin`
in one of `add`'s operands doesn't guarantee that the whole sum reduces, especially in presence
of negative steps and lack of `nuw`. I don't think there is a generic legal way to somehow play
around this `umin`.

So the ad-hoc solution is following: if we failed to find an equivalent predicate that is invariant
during first `MaxIter` iterations, and `MaxIter = umin(a, b, c...)`, try to find solution for at least one
of `a`, `b`, `c`... Because they all are `uge` than `MaxIter`, whatever is true during `a (b, c)` iterations
is also true during `MaxIter` iterations.

Differential Revision: https://reviews.llvm.org/D140456
Reviewed By: nikic