constant expressions evaluator should handle local let-expressions

[anton-trunov]

For instance,

script;

fn main() -> bool {
    let result: u64 = 0 + 1 + 2 + 3 + 4 + 5;

    log(result);

    true
}

currently produces the following IR:

script {
    entry fn main() -> bool, !1 {
        entry():
        v0 = const u64 0, !2
        v1 = const u64 1, !3
        v2 = add v0, v1, !4
        v3 = const u64 2, !5
        v4 = add v2, v3, !6
        v5 = const u64 3, !7
        v6 = add v4, v5, !8
        v7 = const u64 4, !9
        v8 = add v6, v7, !10
        v9 = const u64 5, !11
        v10 = add v8, v9, !12
        v11 = const u64 0
        log u64 v10, v11, !16
        v12 = const bool true, !17
        ret bool v12
    }
}

but one would expect no add instructions for the script shown above.

[xunilrj]

I believe I found some limitations in the "const-eval" module. I will create a PR to address them first.

[xunilrj]

@anton-trunov, I was unsure if changing the IR generation would be beneficial in this case. So I improved the constcombine optimization pass. See #4780

Even if you think the IR generation is a must, I would merge #4780.

Any thoughts?

[xunilrj]

The optimizer pass solution is done. I will experiment with const-eval-uating expression before IR generation.