[simd] Introduce SIMD types (as classes)

[simd] Introduce SIMD types (as classes)

- Introduce a proper bit for SIMD primitive values.
- Introduce constructors for individual SIMD types. These are currently just classes, which seems good enough for now, given that we always have exactly one global map per SIMD type.

The only problem with using class types for SIMD is that a SIMD constant won't be a subtype of its specific type, only of the general SIMD type. But until we actually introduce SIMD constants into the compiler that shouldn't matter.

R=jarin@chromium.org
BUG=

Committed: https://crrev.com/a60f19220aadbe56bc34d2a4c040e75b2a5833fb
Cr-Commit-Position: refs/heads/master@{#30294}

[Jarin, 2015-08-20 17:58:44]

lgtm

[rossberg, 2015-08-21 09:53:59]

The CQ bit was checked by rossberg@chromium.org

[commit-bot: I haz the power, 2015-08-21 09:54:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303863002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303863002/1

[commit-bot: I haz the power, 2015-08-21 10:49:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-21 10:49:45]

Patchset 1 (id:??) landed as
https://crrev.com/a60f19220aadbe56bc34d2a4c040e75b2a5833fb
Cr-Commit-Position: refs/heads/master@{#30294}