[wasm] Cleanup Ifdefs for SIMD ops

Bug: V8:6020
Change-Id: I4a244dce01a3f85d6e85d3c890c0da3d1f39ef16
Reviewed-on: https://chromium-review.googlesource.com/807350Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49879}