Note that a ConstantExpr can be of any type, so classof() in most
other subclasses (that discriminate by type) have to check that it
is also not a ConstantExpr. This is why isConstantExpr() is needed.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2890 91177308-0d34-0410-b5e6-96231b3b80d8