Port 4cbec82c

Original Commit Message:

    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=titzer@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I320373dfe4734b27b85217a829bf574bc7b90a2d
Reviewed-on: https://chromium-review.googlesource.com/c/1273825Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56536}