x86: drop most OPERAND_TYPE_* (and rework the rest)

With the general use of C99 there's no need anymore to have i386-gen
produce these. For more frequently used ones introduce local #define-s,
while others are simply spelled out directly. While doing this move
some static constants into more narrow scopes.

Note that as a "side effect" this corrects type_names[]'es imm8s entry.