Restructured the code a little, to allow getting both the mir::Body and coverage graph.
Per Mark's recommendation at: https://github.com/rust-lang/rust/pull/78963#issuecomment-725790071
As discussed in PR #78267, for example: * https://github.com/rust-lang/rust/pull/78267#discussion_r515404722 * https://github.com/rust-lang/rust/pull/78267#discussion_r515405958