- Renames UnallocatedOperandenum values to reflect the fact that there are
multiple FP machine representations.
- Renames enum values for RegisterAllocatorVerifier enum.
- Template-izes DefineAsFixed and UseFixed methods to handle multiple FP
register types.

BUG=v8:4124

Review-Url: https://codereview.chromium.org/2017733002
Cr-Commit-Position: refs/heads/master@{#36558}