Missing optimization: fold `max(b, a + 1)` to `b` when `a < b`

[XChy]

Alive2 proof: https://alive2.llvm.org/ce/z/V82mt-

Description:

long src(long a, long b) {
    if(a < b){
        return max(b, a + 1);
    }else{
        return 0;
    }
}

can be folded to:

long tgt(long a, long b) {
    if(a < b){
        return b;
    }else{
        return 0;
    }
}

Similarly, it also applies to other cases like a > b, min(b, a + 1) and so on.

Real-world motivation

This snippet of IR is derived from redis/src/db.c@readGetKeys (after O3 pipeline).
The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR, see also:https://godbolt.org/z/YaEYnedYW

Let me know if you can confirm that it's an optimization opportunity, thanks.

[nikic]

I don't think the subtraction is really relevant here. We can optimize the max away regardless of how it is used: https://alive2.llvm.org/ce/z/V82mt-