Instead of loading the address both the limit and top pointers, rely on the
property that the limit pointer is always directly after the top pointer so that
it can be loaded with the limit pointer's address plus a fixed offset.

This generates smaller code and reduces the number of registers required by the
allocation sequence by one.

LOG=N
R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2605043002
Cr-Commit-Position: refs/heads/master@{#41975}