[RTGTest] Improved register representation

[maerhart]

• Allows fixed registers to be pure and constant-like (thus they can CSE, etc.)
• Ops representing fixed registers now fold to an attribute that is of the register type instead of i32 which makes handling in the elaboration pass easier.
• Storing the allowed regs with the op allows the frontend to exclude registers if desired
• If storing a list of attribute pointers with each virtual reg op becomes a performance problem we can change it to allow an empty list to mean any register of the dialect defining the register type is allowed (which the dialect tells us via a dialect interface). We could also use that dialect interface to store the allowed regs as a bitvector with the op then, essentially making it the same as now from a performance perspective (modulo an interface lookup).

Bit unfortunate that CAPI and Python bindings are not autogenerated for types and attributes as they are for ops.