arch: Use finditer in the (Sub)OperandList classes.

arch: Use finditer in the (Sub)OperandList classes.

This method returns an iterator which goes through all the
non-overlapping matches for the given RE, without having to hand code
that same behavior with the more basic "search" method.

Change-Id: I4c4d95cfc8f72125566222aebb56604c3e9e2b03
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35817
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>