Fixed register d27 wasn't used in code generation, so remove it and rename the
remaining fixed registers. Also, remove some left over Crankshaft comments.

Change-Id: I971069c668a597900b1a0c4b64736103a78dab14
Reviewed-on: https://chromium-review.googlesource.com/968426Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#52225}