port f4c079d4 (r30107).

original commit message:

    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.

BUG=

Review URL: https://codereview.chromium.org/1286313003

Cr-Commit-Position: refs/heads/master@{#30241}