LiftoffRegister is an abstraction over Register and DoubleRegister.
Many functions in Liftoff do not need to distinguish GP and FP
registers. LiftoffRegister allows to implement most functions
generically. Accessors allow to convert them back to Register or
DoubleRegister.
Both register types are represented in a unified index space, which
allows to extend this concept to more register types and implement
aliasing in a straight-forward manner.
Many functions currently only implement the GP case. FP will be added
later.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I043b787bc09dd1a06506ad515eb325b8ea19746d
Reviewed-on: https://chromium-review.googlesource.com/793390Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49780}