Disassembler: select table based on VEX prefixes

Disassembler: select table based on VEX prefixes

We can use the new VEX prefixes to select into a large table of new
opcode spaces.  Since the table is (currently) sparse, add logic so we
don't end up producing tons of empty tables for no good reason.

This is also necessary since VEX is likely to reuse opcode bytes that
would appear as prefixes at some point, which would cause conflicts
with the regular tables.