add a layer of accessors around the Value::SubClassData member, and use

a convention (shadowing the setter with private forwarding function) to
prevent subclasses from accidentally using it.

This exposed some bogosity in ConstantExprs, which was propaging the
opcode of the constant expr into the NUW/NSW/Exact field in the
getWithOperands/getWithOperandReplaced methods.

llvm-svn: 92239