- Eliminates S32x4Shuffle, S16x8Shuffle opcodes. All shuffles are subsumed
  by S8x16Shuffle. This aligns us with the latest WASM SIMD spec.

LOG=N
BUG=v8:6020

Review-Url: https://codereview.chromium.org/2923103003
Cr-Commit-Position: refs/heads/master@{#45929}