[turbofan] Introduce conditional builders into AstGraphBuilder.

[turbofan] Introduce conditional builders into AstGraphBuilder.

This re-introduces the long lost concept of conditional builder methods
into the AstGraphBuilder that are allowed to return {nullptr} when a
certain optimization does not apply. This can be used to separate our
optimizations in the graph builder clearly from code required for
correctness.

R=bmeurer@chromium.org
BUG=v8:4513
LOG=n

Committed: https://crrev.com/335a8fc3321f78d85b96f5b6ada0963d59e7d58a
Cr-Commit-Position: refs/heads/master@{#31694}

[Michael Starzinger, 2015-10-30 14:29:51]

Description was changed from

==========
[turbofan] Introduce conditional builders into AstGraphBuilder.

This re-introduces the long lost concept of conditional builder methods
into the AstGraphBuilder that are allowed to return {nullptr} when a
certain optimization does not apply. This can be used to separate our
optimizations in the graph builder clearly from code required for
correctness.

R=bmeurer@chromium.org
==========

to

==========
[turbofan] Introduce conditional builders into AstGraphBuilder.

This re-introduces the long lost concept of conditional builder methods
into the AstGraphBuilder that are allowed to return {nullptr} when a
certain optimization does not apply. This can be used to separate our
optimizations in the graph builder clearly from code required for
correctness.

R=bmeurer@chromium.org
BUG=v8:4513
LOG=n
==========

[Benedikt Meurer, 2015-10-30 14:34:15]

Oh nice. LGTM!

[Michael Starzinger, 2015-10-30 15:03:23]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-10-30 15:03:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1414723004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1414723004/20001

[commit-bot: I haz the power, 2015-10-30 15:58:50]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-30 15:59:12]

Patchset 2 (id:??) landed as
https://crrev.com/335a8fc3321f78d85b96f5b6ada0963d59e7d58a
Cr-Commit-Position: refs/heads/master@{#31694}