This adds support for i64.shl, i64.shr_s, and i64.shr_u.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ibc1982064ff3c9f3a494b5fa329841a0d3bc7bce
Reviewed-on: https://chromium-review.googlesource.com/964844
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52022}