[llvm] [SDAG] Use shifts if ISD::MUL is illegal when lowering ISD::CTPOP (PR #86505)

[Craig Topper via llvm-commits]

topperc wrote:

> You can do this with fewer iterations by doubling the shift amount each time, e.g. for 64 bits:
> 
> ```
> v += v << 8;
> v += v << 16;
> v += v << 32;
> return v >> 56;
> ```

Or

```
v += v << 32
v += v << 16
v += v << 8
return v >> 56
```

But I think either would work.

https://github.com/llvm/llvm-project/pull/86505