- Lane indices are no longer required to be integers. Add index coersion for loads/stores
 - Give shift operators masking shift count semantics
 - Throw type/range errors instead of runtime asserts.

BUG=v8:4963
LOG=N

R=bbudge@chromium.org, bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/1965443003
Cr-Commit-Position: refs/heads/master@{#36402}