Many methods which currently receive a {ValueType} really just need to
know the register class. Thus introduce a {RegClass} enum with values
{kGpReg} and {kFpReg}, and change some methods to use it. For most
methods, only the {kGpReg} case is implemented for now. Support for
{kFpReg} will be added later.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I1db793c59270862c4e1b877e6e4bd81020a650f8
Reviewed-on: https://chromium-review.googlesource.com/793159Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49670}