Remove per-instruction functions and data structures

Improve static code/data size by:

 1) Removing the per-instruction visitor functions. The generic Visit()
    function can be overridden instead to intercept individual instructions, as
    shown in the existing custom disassembler example.
 2) Remove the array of instruction forms. Instead infer the forms from the
    decoder tree defined by kDecodeMapping.

As it's now unused, remove the INSTRUCTION_VISITOR_LIST define too.