Document non-use of certain operand types

Document non-use of certain operand types

Operand class enum values not used in the syntax table:
- Image channel order: only used to describe a return value
- Image channel type: only used to describe a return value
- "Image operands": used to make a section in the spec
  to describe values used to construct an optional image id,
  but does not itself appear in the syntax table.

Removes some TODOs from opcode.cpp.