Minor refactoring to introduce kMaxRegisters instead of relying on
the fact that kMaxGpRegisters <= kMaxFpRegisters.

Bug: v8:8562
Change-Id: If8221a799199c62d5262cfad762489a5088351c7
Reviewed-on: https://chromium-review.googlesource.com/c/1403120Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58687}