- Simd Scalar lowering should be conditionally disabled if the architecture has a native SIMD implementation.
 - Enable scalar lowering tests on all architectures instead of only x64.

R=bbudge@chromium.org, aseemgarg@chromium.org

Review-Url: https://codereview.chromium.org/2514663002
Cr-Commit-Position: refs/heads/master@{#41160}