Up to now, each architecture defined all Register types as structs,
with lots of redundancy. An often found comment noted that they cannot
be classes due to initialization order problems. As these problems are
gone with C++11 constexpr constants, I now tried making Registers
classes again.
All register types now inherit from RegisterBase, which provides a
default set of methods and named constructors (like ::from_code,
code(), bit(), is_valid(), ...).
This design allows to guarantee an interesting property: Each register
is either valid, or it's the no_reg register. There are no other
invalid registers. This is guaranteed statically by the constexpr
constructor, and dynamically by ::from_code.

I decided to disallow the default constructor completely, so instead of
"Register reg;" you now need "Register reg = no_reg;". This makes
explicit how the Register is initialized.

I did this change to the x64, ia32, arm, arm64, mips and mips64 ports.
Overall, code got much more compact and more safe. In theory, it should
also increase performance (since the is_valid() check is simpler), but
this is probably not measurable.

R=mstarzinger@chromium.org

Change-Id: I5ccfa4050daf4e146a557970e9d37fd3d2788d4a
Reviewed-on: https://chromium-review.googlesource.com/650927Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47847}