Currently {{seq}} and {{boolean}} tags are considered "safe" for elimination of checked {{if}}.
Consider this code:
`
(defn foo [x] (if x true nil))
(defn bar [x] (if (foo x) 1 2))
`
In this case {{foo}} is inferred as 1. }}, but {{bar}} will involve a call to {{cljs.core.truth_}}. It is safe to eliminate the checked if in this case as {{clj-nil}} will be interpreted by JavaScript correctly as {{false}}.
Note: CLJS-2869 has some utility code that makes this ticket easier to implement.
