[turbofan] Compute proper type for globals with "constant type".

[turbofan] Compute proper type for globals with "constant type".

For property cells with cell type kConstantType, we can compute an
appropriate JavaScript type based on the current value of that cell.
Numbers cannot use Type::Of here, because the type might be too precise,
so we handle smi and heap number specially.

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

Committed: https://crrev.com/fd4e0dc2ce85281a3f713eee4e345525126460d7
Cr-Commit-Position: refs/heads/master@{#31171}

[Benedikt Meurer, 2015-10-08 06:04:48]

Hey Jaro,

Fix open TODO from initial commit.
Please take a look.

Thanks,
Benedikt

[Jarin, 2015-10-08 08:25:09]

lgtm

[Benedikt Meurer, 2015-10-08 08:48:57]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-10-08 08:49:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1400483002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1400483002/1

[commit-bot: I haz the power, 2015-10-08 08:59:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 08:59:22]

Patchset 1 (id:??) landed as
https://crrev.com/fd4e0dc2ce85281a3f713eee4e345525126460d7
Cr-Commit-Position: refs/heads/master@{#31171}