PPC: [simd.js] Single SIMD128_VALUE_TYPE for all Simd128Values.

PPC: [simd.js] Single SIMD128_VALUE_TYPE for all Simd128Values.

Port f4c079d450a5990639b295d40a3d1663d70412d6

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.

R=bmeurer@chromium.org, jyan@ca.ibm.com, dstence@us.ibm.com, joransiu@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1312513004

Cr-Commit-Position: refs/heads/master@{#30392}