unreachable code error when there is a defer with 0 block exit paths

[andrewrk]

Extracted from #110.

Example code:

const std = @import("std");

test "example" {
    defer bar();
    defer foo();
}

fn foo() noreturn {
    while (true) {}
}
fn bar() void {}

expected: error: unreachable code

actual: code compiles