PPC: [turbofan] Avoid unnecessary write barriers and improve code generation.

PPC: [turbofan] Avoid unnecessary write barriers and improve code generation.

Port 1e2770123bd5c1cf7631bf11be846f0fbd884c08

Original commit message:
    Avoid write barriers when storing values in the root set, and use
    cheaper write barriers for storing maps or tagged pointers. Also
    improve the generated code for write barriers, utilizing the out
    of line code mechanism that is available to TurboFan backends,
    which moves the unlikely case out of the hot path.

R=bmeurer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, dstence@us.ibm.com
BUG=

Committed: https://crrev.com/e2f4f04f20ac066e935ab5a95ba35009b430188d
Cr-Commit-Position: refs/heads/master@{#31927}

[MTBrandyberry, 2015-11-10 19:36:41]

The CQ bit was checked by mbrandy@us.ibm.com to run a CQ dry run

[commit-bot: I haz the power, 2015-11-10 19:36:51]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1431923003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1431923003/1

[michael_dawson, 2015-11-10 19:55:10]

On 2015/11/10 19:36:51, commit-bot: I haz the power wrote:
> Dry run: CQ is trying da patch. Follow status at
>  https://chromium-cq-status.appspot.com/patch-status/1431923003/1
> View timeline at
>  https://chromium-cq-status.appspot.com/patch-timeline/1431923003/1

lgtm

[michael_dawson, 2015-11-10 19:55:57]

On 2015/11/10 19:36:51, commit-bot: I haz the power wrote:
> Dry run: CQ is trying da patch. Follow status at
>  https://chromium-cq-status.appspot.com/patch-status/1431923003/1
> View timeline at
>  https://chromium-cq-status.appspot.com/patch-timeline/1431923003/1

lgtm.

[commit-bot: I haz the power, 2015-11-10 19:56:13]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-10 19:56:13]

Dry run: This issue passed the CQ dry run.

[MTBrandyberry, 2015-11-10 19:57:41]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-11-10 19:57:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1431923003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1431923003/1

[commit-bot: I haz the power, 2015-11-10 19:59:19]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-10 19:59:29]

Patchset 1 (id:??) landed as
https://crrev.com/e2f4f04f20ac066e935ab5a95ba35009b430188d
Cr-Commit-Position: refs/heads/master@{#31927}