- Use correct prefixes for SIMD/Atomics ops
 - S128 LoadMem/StoreMem should not use 0xc0/0xc1 opcodes, these are now
 being used for sign extension
 - S128 LoadMem/StoreMem should use prefixed opcodes

BUG=v8:6020

Review-Url: https://codereview.chromium.org/2943773002
Cr-Commit-Position: refs/heads/master@{#46016}