[llvm-bugs] [Bug 43198] New: LLVM should better understand subtraction of `zext i1`s

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43198

            Bug ID: 43198
           Summary: LLVM should better understand subtraction of `zext
                    i1`s
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: me22.ca at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

I found this trying to make a better implementation of Rust's `Ord::cmp` for
integers.

C++ repro https://godbolt.org/z/tL0-oW
```
int spaceship(int a, int b) {
    return (a > b) - (a < b);
}

bool my_lt(int a, int b) {
    return spaceship(a, b) == -1;
}
```

`my_lt` there should be foldable down to just `a < b`, but that doesn't happen
```
define dso_local zeroext i1 @_Z5my_ltii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = icmp sgt i32 %0, %1
  %4 = zext i1 %3 to i32
  %5 = icmp slt i32 %0, %1
  %6 = zext i1 %5 to i32
  %7 = sub nsw i32 %4, %6
  %8 = icmp eq i32 %7, -1
  ret i1 %8
}
```

(And running it through `opt` again doesn't help either,
https://godbolt.org/z/-hl5H0)

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