Take advantage of LLVM range metadata for enum discriminants

According to [LLVM Language Reference Manual](http://llvm.org/docs/LangRef.html), "range metadata may be attached only to loads of integer types. It expresses the possible ranges the loaded value is in".

Clang uses it to let LLVM know that even if bool is represented as i8 in LLVM, it can be only 0 or 1. See http://llvm.org/bugs/show_bug.cgi?id=12251
