let_chains: Refactor parse_{if,while}_expr a bit.
let_chains: Refactor parse_{if,while}_expr a bit.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust