Pure refactoring CL. Introduce a symbolic name for the register
used to hold the function index when calling the lazy compile stub.
This makes it easier to see this contract when looking at the
macro assembler.

R=ahaas@chromium.org
CC=clemensh@chromium.org

Change-Id: I714f978883ced001a1435338dcefd96744bfb2ae
Reviewed-on: https://chromium-review.googlesource.com/c/1273099
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56516}