[turbofan] Slightly refactor JSLoadGlobal specialization.

[turbofan] Slightly refactor JSLoadGlobal specialization.

Reduce code duplication somewhat restructure the code to make
it easier to spot the code paths for the different property
cell types.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Committed: https://crrev.com/113b78e9e6646594949f8b8f040337e55160927c
Cr-Commit-Position: refs/heads/master@{#31589}

[Jarin, 2015-10-26 21:33:34]

lgtm. (even though I do not really find the new version more readable)

[Benedikt Meurer, 2015-10-27 03:09:33]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-10-27 03:09:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1413383004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1413383004/1

[commit-bot: I haz the power, 2015-10-27 04:40:11]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-27 04:40:45]

Patchset 1 (id:??) landed as
https://crrev.com/113b78e9e6646594949f8b8f040337e55160927c
Cr-Commit-Position: refs/heads/master@{#31589}