Port 9e995e12
Port 408f252b

  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=bjaideep@ca.ibm.com, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I2e87efc8790290c64fd6c0a2d093326710b30ed3
Reviewed-on: https://chromium-review.googlesource.com/658065Reviewed-by: Jaideep Bajwa <bjaideep@ca.ibm.com>
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#47933}