arch: Clean up the __init__s in (Sub)OperandList.

arch: Clean up the __init__s in (Sub)OperandList.

These had a lot of for loops and ifs and nesting. Python lets you avoid
that, which makes the code easier to read and more intuitive to
understand.

Change-Id: I576bf1de9e5b2268717a535ca42f2db669d83ed2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35818
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>