[llvm] [CodeGenPrepare] Folding `urem` with loop invariant value as remainder (PR #96625)

[via llvm-commits]

goldsteinn wrote:

Okay fairly certain I found the bug. Reproduced (ish) with a cross build and think the issue is the `Start > RemAmt` case wasn't being correctly handled. Fix is:
```
  Start = simplifyURemInst(Start, RemAmt, *DL);
  if (!Start)
    return false;
```

https://github.com/llvm/llvm-project/pull/96625