Port ec132e05

Original commit message:

    (GcStress failure was unrelated.)

    At one time, we hoped to generate the same code for different
    native contexts. But in truth, much performance comes from optimizing
    on the native context. Now we abandon this pathway.

R=mvstanton@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2401043002
Cr-Commit-Position: refs/heads/master@{#40092}