This cleanup is the result of trying to modify the `Assembler::addrmod1` method
and realising it's very easy to break it. It handles three groups of
instructions with different operands and uses `r0` when a register is not used:

- General case:            rd, rn, (rm|rm shift #imm|rm shift rs)
- Comparison instructions:     rn, (rm|rm shift #imm|rm shift rs)
- Move instructions        rd,     (rm|rm shift #imm|rm shift rs)

Let's use `no_reg` instead of `r0` with explicit checks and assertions so that
it's clear this method is used with multiple types of instructions.
Additionaly, keep the order of operands as "rd", "rn", "rm".

As drive-by fixes, I've taken the opportunity to add a few helper methods to the
`Operand` class.

Bug: 
Change-Id: If8140d804bc90dea1d3c186b3cee54297f91462a
Reviewed-on: https://chromium-review.googlesource.com/531284
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45949}