[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=

Review URL: https://codereview.chromium.org/1303863002

Cr-Commit-Position: refs/heads/master@{#30294}