arch: Split the operand types out of the ISA parser.

arch: Split the operand types out of the ISA parser.

These conceptually go together and don't depend on any other parts of
the parser.

Change-Id: Ia8bff0d0ec210bdeeb080808968faf9528ee03dd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35278
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>