[haskell-llvm] make IsType superclass of IsConst

[Henning Thielemann]

In LLVM.Core.CodeGen I find

> -- XXX merge with IsArithmetic?
> class IsConst a where
>     constOf :: a -> ConstValue a


I do not know, whether all arithmetic types allow (and will allow in 
future) initialization with constants, however I think that at least 
IsType should be a superclass of IsConst.