https://pastebin.com/fDvgWgHe

After having a chat with Dougall Johnson (https://twitter.com/dougallj/status/1534213050944802816), we realized that __clzll works with zero arguments per documentation:

https://developer.arm.com/documentation/101028/0009/Data-processing-intrinsics

```
Returns the number of leading zero bits in x. When x is zero it returns the argument width, i.e. 32 or 64.
```

Codegen improves https://godbolt.org/z/ebadf717Y

Thus we can use a little bit different construction not involving CLS but using more understandable CLZ and removing some operations.

PiperOrigin-RevId: 453879080
Change-Id: Ie2d7f834f63364d7bd50dd6a682c107985f21942