[llvm] [ConstantRange] Bail out early in unsignedMulMayOverflow for wide integer types (PR #192275)

[via llvm-commits]

Firebear518 wrote:

Suggestion: Simplify the early-out conditions based on LZ

I noticed that we currently have two separate if statements for the fast paths (LZ >= BitWidth and LZ < BitWidth - 1). Since both cases ultimately just return *this * RHS, I think we can combine them into a single condition to remove a branch and reduce code duplication.

Current:
```C++
if (LZ >= BitWidth) {
  Overflow = false;
  return *this * RHS;
}

if (LZ < BitWidth - 1) {
  Overflow = true;
  return *this * RHS;
}
```



Here is what I have in mind:

```C++
if (LZ != BitWidth - 1) {
  Overflow = (LZ < BitWidth - 1);
  return *this * RHS;
}
```
This perfectly covers both the "no-overflow" and "guaranteed overflow" cases while keeping the borderline case (LZ == BitWidth - 1) for the slow path.

What do you think about this refactoring? I'd love to hear your thoughts!

https://github.com/llvm/llvm-project/pull/192275