ISA parser: Simplify operand type handling.

This change simplifies the code surrounding operand type handling and makes it
depend only on the ctype that goes with each operand type. Future changes will
allow defining operand types by their ctypes directly, convert the ISAs over
to that style of definition, and then remove support for the old style. These
changes are to make it easier to use non-builtin types like classes or
structures as the type for operands.