Incorrect MIR built for loops inside expressions.

[solson]

Test case

#![feature(rustc_attrs)]

#[rustc_mir(pretty = "foo.mir")]
fn foo() -> [String; 2] {
    [String::from("test"), loop {}]
}

fn main() {
    println!("{:?}", foo());
}

Generated MIR for foo

fn() -> [collections::string::String; 2] {
    let mut tmp0: collections::string::String;
    let mut tmp1: collections::string::String;

    bb0: {
        tmp0 = core::convert::From::from(const "test") -> [return: bb3, unwind: bb2];
    }

    bb1: {
        return;
    }

    bb2: {
        drop tmp0;
        resume;
    }

    bb3: {
        tmp1 = ();
        goto -> bb3;
    }
}

Notice how the function foo assigns an rvalue () to tmp1 which has type collections::string::String. This seems to not affect the built executable since tmp1 = () is a no-op, but MIR should probably be type-correct at this stage.

[solson]

Closing in favor of #30636.