Some of these cases currently don't occur in practice, but are included for
completeness, and to avoid having to add them later as branch coverage and
MC/DC coverage start building more complex expressions.
When we try to extract coverage-relevant spans from MIR, sometimes we see MIR
statements/terminators whose spans cover the entire function body. Those spans
tend to be unhelpful for coverage purposes, because they often represent
compiler-inserted code, e.g. the implicit return value of `()`.
