LiftoffRegister::gp() and LiftoffRegister::fp() are constexpr.
Therefore, VRegister::from_code() needs to be constexpr as well.

Bug: chromium:819294
Change-Id: I5a75d6ae0dc79fce0a42a45c5f7928aa61ac5520
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2922887Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Stephan Hartmann <stha09@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#74932}