rustc_typeck: use IndexSet in UniquePredicates
rustc_typeck: use IndexSet in UniquePredicates

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