Elide IIFEs for single-expression function bodies

[wcjohnson]

When a function body consists of just an expression that would be wrapped in an IIFE, the IIFE can safely be elided; this will generate slightly better code.

Lscdiag: https://wcjohnson.github.io/lscdiag/?v=1&s=eMOafcKQw4FOw4MwEER_ZcOrS8KpFMKiwqBSBMKWWsO1BDc4cAxRw6QmwptoGzvCrmzDhwVFw6bDm8KxEXBCw53Ci8OHw7LDs2hnZsOWw5HCu8KbDFrDhmdWK3FKw6d-wq_DkMKJVjjCkW7DjcKxwrtpN2fCtSnDosKsw6nDrl7DocOtwrp_w7DCmnFnJgzDmSViZlZPwqbDgcKkwqRtGGdawpF7EsOnw6HDkWjDtcKKw47DkcOYw5srw7sjVnDCvXsbAWbDiMOzwrzDkwYGw7zCgCEDLyR4wqARfkHCnsKoNMOUw4EAwostLMOrwrrCj8KudcK9w7x7woo2w6VQw7HChMO4T8KyL8KHIzZuw4HDocOfDcO8ClBaw6TDoEPDuh9Yw4w1UVrDm8KJQ2zCpyzCssKiw4rCmEUZTcKwfcOWbcOMNE5SwqbDvHrDrMKow7_DrkrCqxNJNDHCpxQOwq1jIURAUlTCvMK8UFQVw4IXw6RAwoER#