Don't remove unreachable statements at HIR lowering

[lrh2000]

This is a legal code segment:

fn foo() -> i32 {
    return 1;
    let a = 1;
    a
}

And this is illegal:

fn foo() -> i32 {
    return 1;
    let a = 1.5;
    a
}

So we shouldn't remove unreachable statements at HIR lowering. We can issue warnings for them, but they need to be kept so that their types can be checked.

(Original in #364)

[philberty]

bors r+

[bors]

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