x86/APX: with REX2 map 1 doesn't "chain" to maps 2 or 3

Don't wander into three_byte_table[] when REX2 is present.

While there also eliminate related confusion when accessing
dis386_twobyte[]: There's nothing 3-byte-ish involved there. Dropping
the odd variable gets things better in sync with 1-byte handling as
well.