Missed Optimization: Rewrite sub(x, select(x<1 ? -1 : 1)) == 0 → (x == -1) || (x == 1)

```llvm
define i1 @src(i32 %arg0) {
%v0 = icmp slt i32 %arg0, 1
%v1 = select i1 %v0, i32 -1, i32 1
%v2 = sub nsw i32 %arg0, %v1
%v3 = icmp eq i32 %v2, 0
ret i1 %v3
}

define i1 @tgt(i32 %arg0) {
%v0 = icmp eq i32 %arg0, -1
%v1 = icmp eq i32 %arg0, 1
%v2 = or i1 %v0, %v1
ret i1 %v2
}
```

alive2: https://alive2.llvm.org/ce/z/c-_auN
godbolt: https://godbolt.org/z/17b7bh598
Pattern found in: https://github.com/dtcxzyw/llvm-opt-benchmark/blob/main/bench/ffmpeg/optimized/pvq.ll