Reland^2 "Revamp JIT GC."

This reverts commit 030a225ade89e2eaf34ac0072b6b1f9ac02ddda7.

Reason for revert: Prevent zombie code being added in the middle of a
JIT GC.

Change-Id: I89157e4d4130896665a11a930d87635f4883db79
8 files changed