Handle MIR FOR CTFE functions

[bbyalcinkaya]

rustc produces duplicate functions for compile-time function evaluation (CTFE).

Example: ui/attrs-resolution.mir

fn FooEnum::Bar(_1: i32) -> FooEnum {
    let mut _0: FooEnum;                 // return place in scope 0 at ui/attrs-resolution.rs:5:5: 5:8

    bb0: {
        _0 = FooEnum::Bar(move _1);      // scope 0 at ui/attrs-resolution.rs:5:5: 5:8
        return;                          // scope 0 at ui/attrs-resolution.rs:5:5: 5:8
    }
}

// MIR FOR CTFE
fn FooEnum::Bar(_1: i32) -> FooEnum {
    let mut _0: FooEnum;                 // return place in scope 0 at ui/attrs-resolution.rs:5:5: 5:8

    bb0: {
        _0 = FooEnum::Bar(move _1);      // scope 0 at ui/attrs-resolution.rs:5:5: 5:8
        return;                          // scope 0 at ui/attrs-resolution.rs:5:5: 5:8
    }
}

[yanliu18]

Merge to issue #65