x86: introduce operand type "instance"

x86: introduce operand type "instance"

Special register "class" instances can't be combined with one another
(neither in templates nor in register entries), and hence it is not a
good use of resources (memory as well as execution time) to represent
them as individual bits of a bit field.

Furthermore the generalization becoming possible will allow
improvements to the handling of insns accepting only individual
registers as their operands.