- This won't save a bit, since the enum is not a flags enum and there is
  still room for another value.
- While it may be possible to eliminate this value, the code to init
  and check for invalid becomes much more complex. Unallocated operands
  are currently created with invalid vreg code, so we also have to check
  the policy.

BUG=v8:6325

Review-Url: https://codereview.chromium.org/2904603004
Cr-Commit-Position: refs/heads/master@{#45534}