constants using operators such as cast, getelementptr, add, shl, etc.
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.
llvm-svn: 2891
now knows how to write out a constant, not the constants themselves. This is
fixed due to the move of the AsmWriter to the VMcore library.
llvm-svn: 2297
