[llvm] [SDAG] Simplify is-power-of-2 codegen (PR #72275)

[Tavian Barnes via llvm-commits]

tavianator wrote:

> Do you have an alive2 proof please?

https://alive2.llvm.org/ce/z/kttm55

```
----------------------------------------
define i1 @src(i32 %x) {
0:
  %cnt = ctpop i32 %x
  %r = icmp eq i32 %cnt, 1
  ret i1 %r
}
=>
define i1 @tgt(i32 %x) {
0:
  %y = sub i32 %x, 1
  %z = xor i32 %x, %y
  %r = icmp ugt i32 %z, %y
  ret i1 %r
}
Transformation seems to be correct!
```

https://github.com/llvm/llvm-project/pull/72275