typeck/pat.rs: simplify `check_pat_walk`.
typeck/pat.rs: simplify `check_pat_walk`.

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