[llvm] [SCEV] Remove udiv exact logic (PR #195840)

[Ryan Buchner via llvm-commits]

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bababuck wrote:

I don't think this is equivalent to the logic in `getUDivExactExpr()` since this is only implemented for constants (as guarded by the if statement on line 3564):
```
 if (const SCEVConstant *RHSC = dyn_cast<SCEVConstant>(RHS)) {
```

https://github.com/llvm/llvm-project/pull/195840