chore: fix a bad way to using machine

[mohanson]

The old codes:

let value = &machine.registers()[i.rs2()]; // immutable borrow machine
update_register(machine, i.rd(), value.clone()); // mutable borrow machine in first argument, and immutable borrow in third argument "value"

[nervos-bot]

@xxuejie is assigned as the chief reviewer

[xxuejie]

I'm not sure if the code here makes a difference, value.clone() part in the old code already kills the immutable borrow, so the second line here only has a mutable borrow on the machine type.

Personally I have no problems writing the code either way, but the point I want to make is: if we do want to make the change, there're many other places in this module where the code is written in a similar style, so we either keep them unchanged, or change all of the code together to keep a unified style.

Thoughts?

[mohanson]

I think this is the only place in this module that needs to be modified.

Rust 1.35.0 stable gives warnings see bellows:

warning: cannot borrow `*machine` as mutable because it is also borrowed as immutable
   --> src\instructions\execute.rs:643:29
    |
642 |             let value = &machine.registers()[i.rs2()];
    |                          ------- immutable borrow occurs here
643 |             update_register(machine, i.rd(), value.clone());
    |                             ^^^^^^^          ----- immutable borrow later used here
    |                             |
    |                             mutable borrow occurs here
    |
    = note: #[warn(mutable_borrow_reservation_conflict)] on by default
    = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
    = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

[xxuejie]

Ah I see, I didn't know this behavior change, in this case this is all good, I still want to change all the other parts as a unified style change but we can leave it till another time. Thanks