There's no need to have one InstanceType per SIMD primitive type (this
will not scale long-term).  Also reduce the amount of code duplication
and make it more robust wrt adding new SIMD types.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1273353003

Cr-Commit-Position: refs/heads/master@{#30107}