Emit mementos in crankshaft.

Emit mementos in crankshaft. Right now, it's controlled by asking an
allocation site if it's "undecided" yet. A flag
"pretend-pretenuring-uncertainty" can be used to force all allocation
sites to masquerade as undecided for testing purposes.

Crankshaft mementos can be turned on for:
* Array and object literals
* Array and object constructor calls (if pretenuring_call_new is
  turned on)

R=hpayer@chromium.org

[mvstanton, 2014-05-16 12:49:21]

Hi Hannes,
Here is the change we've discussed. I'll follow up shortly with binary op
strings also ports.
thx,
--Michael

[mvstanton, 2014-05-16 16:38:10]

(Be sure to get the latest rev if you want to try it out. There were interesting
simulate/side-effect issues that have been addressed now.
Best,
--Michael)

[mvstanton, 2014-05-28 15:47:29]

Now with a verifying test. Found out this way that string ops won't work without
some thinking/discussion.

[mvstanton, 2014-06-02 07:03:13]

Adding Benedikt...

[mvstanton, 2014-06-04 12:14:29]

Rebased against current tree, allowing experimentation.