Add a PLACEHOLDER code kind.

Add a PLACEHOLDER code kind.

The PLACEHOLDER code kind is used when compiling a code object that has
direct calls to other code objects, but those other code objects do not
yet exist because they have not yet been compiled. It serves as a
placeholder to break the cycle, e.g. in WASM.

R=yangguo@chromium.org
BUG=

Committed: https://crrev.com/e4bcc3363f2db15bbc0ba57b91676ce7f1c4adb1
Cr-Commit-Position: refs/heads/master@{#30348}

[Yang, 2015-08-25 09:40:14]

On 2015/08/24 19:51:52, titzer wrote:

LGTM.

I wonder whether we can know the code kind upfront, even if the code object does
not exist yet. And where do we need the code kind when calling into a code
object that does not yet exist?

[titzer, 2015-08-25 09:44:17]

On 2015/08/25 09:40:14, Yang wrote:
> On 2015/08/24 19:51:52, titzer wrote:
> 
> LGTM.
> 
> I wonder whether we can know the code kind upfront, even if the code object
does
> not exist yet. And where do we need the code kind when calling into a code
> object that does not yet exist?

As discussed in person, these placeholders hold a little bit of data that
identifies what will be called in the future. I use a field in the code object
to encode that little bit of data, but I need the code kind to encode that this
code object is itself a placeholder.

[titzer, 2015-08-25 10:15:58]

The CQ bit was checked by titzer@chromium.org

[commit-bot: I haz the power, 2015-08-25 10:16:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308393003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308393003/1

[commit-bot: I haz the power, 2015-08-25 10:17:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-25 10:17:41]

Patchset 1 (id:??) landed as
https://crrev.com/e4bcc3363f2db15bbc0ba57b91676ce7f1c4adb1
Cr-Commit-Position: refs/heads/master@{#30348}