Cells were needed originally because there was no typed remembered set to
record direct pointers from code space to new space. A previous
CL (https://codereview.chromium.org/2003553002/) already introduced
the remembered set, this CL uses it.

This CL
* stores direct pointers in code objects, even if the target is in new space,
* records the slot of the pointer in typed-old-to-new remembered set,
* adds a list which stores weak code-to-new-space references,
* adds a test to test-heap.cc for weak code-to-new-space references,
* removes prints in tail-call-megatest.js

R=ulan@chromium.org

Review-Url: https://codereview.chromium.org/2045263002
Cr-Commit-Position: refs/heads/master@{#37134}