Get rid of magic open-coded "register numbers"

Get rid of magic open-coded "register numbers"

Get rid of magic open-coded register numbers.  We now keep track of
a total of three different kinds of register numbers: the register
enumeration (regs.h), the x86 register value, and the register flags.
That has all the information we need.

Additionally, do massive revamping of the EA generation code and the
REX generation logic.