Begin refactoring ir_dereference

Begin refactoring ir_dereference

Create separate subclasses of ir_dereference for variable, array, and
record dereferences.  As a side effect, array and record dereferences
no longer point to ir_variable objects directly.  Instead they each
point to an ir_dereference_variable object.

This is the first of several steps in the refactoring process.  The
intention is that ir_dereference will eventually become an abstract
base class.