[turbofan] Various cleanups.

- Decouple JSBuiltinReducer from JSTypedLowering.
- Unify JSTypedLowering::ReduceJSToXXX() lowering.
- Cleanup several includes and forward declarations.
- Unify helper methods.

TEST=cctest
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/797903003

Cr-Commit-Position: refs/heads/master@{#25790}