Use `_shouldCompileJS` to guard precompilation of addon JS.
Use `_shouldCompileJS` to guard precompilation of addon JS.

This makes it simpler to reason about when reviewing the `processedAddonJSFiles`
method, and adds an extension point (still private) allowing addons that rely on
processing addon JS files but do not have `addon/**/*.js` files (where they are
generally shifting things from a node module into their addon path).

jgwhite/ember-cli,jrjohnson/ember-cli,romulomachado/ember-cli,mike-north/ember-cli,mike-north/ember-cli,cibernox/ember-cli,patocallaghan/ember-cli,Turbo87/ember-cli,trentmwillis/ember-cli,buschtoens/ember-cli,cibernox/ember-cli,jgwhite/ember-cli,kanongil/ember-cli,cibernox/ember-cli,ember-cli/ember-cli,kellyselden/ember-cli,HeroicEric/ember-cli,mike-north/ember-cli,kategengler/ember-cli,fpauser/ember-cli,gfvcastro/ember-cli,HeroicEric/ember-cli,patocallaghan/ember-cli,sivakumar-kailasam/ember-cli,asakusuma/ember-cli,rtablada/ember-cli,thoov/ember-cli,sivakumar-kailasam/ember-cli,kanongil/ember-cli,rtablada/ember-cli,gfvcastro/ember-cli,ef4/ember-cli,pzuraq/ember-cli,seawatts/ember-cli,fpauser/ember-cli,seawatts/ember-cli,seawatts/ember-cli,calderas/ember-cli,akatov/ember-cli,sivakumar-kailasam/ember-cli,kanongil/ember-cli,elwayman02/ember-cli,balinterdi/ember-cli,thoov/ember-cli,raycohen/ember-cli,twokul/ember-cli,romulomachado/ember-cli,ef4/ember-cli,jgwhite/ember-cli,akatov/ember-cli,twokul/ember-cli,pzuraq/ember-cli,akatov/ember-cli,kategengler/ember-cli,rtablada/ember-cli,jgwhite/ember-cli,jrjohnson/ember-cli,calderas/ember-cli,gfvcastro/ember-cli,akatov/ember-cli,mike-north/ember-cli,Turbo87/ember-cli,seawatts/ember-cli,sivakumar-kailasam/ember-cli,rtablada/ember-cli,Turbo87/ember-cli,ember-cli/ember-cli,raycohen/ember-cli,patocallaghan/ember-cli,elwayman02/ember-cli,buschtoens/ember-cli,HeroicEric/ember-cli,Turbo87/ember-cli,fpauser/ember-cli,twokul/ember-cli,balinterdi/ember-cli,thoov/ember-cli,kellyselden/ember-cli,cibernox/ember-cli,gfvcastro/ember-cli,trentmwillis/ember-cli,ef4/ember-cli,fpauser/ember-cli,pzuraq/ember-cli,ember-cli/ember-cli,calderas/ember-cli,trentmwillis/ember-cli,calderas/ember-cli,kanongil/ember-cli,trentmwillis/ember-cli,asakusuma/ember-cli,twokul/ember-cli,patocallaghan/ember-cli,thoov/ember-cli,romulomachado/ember-cli,kellyselden/ember-cli,HeroicEric/ember-cli,ef4/ember-cli,pzuraq/ember-cli,romulomachado/ember-cli,kellyselden/ember-cli