This cl also fixes two bugs in the previous code:
    1) JITed functions were not allowed access to the heap because the module instance wasn't correctly synthesized. This wasn't discovered in the previous test.
    2) Decoding of functions with the JITSingleFunction opcode was off by 1 as the length of the opcode wasn't computed correctly.
BUG=5044

Review-Url: https://codereview.chromium.org/2168183002
Cr-Commit-Position: refs/heads/master@{#37957}