This CL adds a Reduction for the DataViewGetInt16 and -Uint16 builtins,
and the corresponding handling in LoadDataViewElement node in the
effect control linearizer.

It also adds tests for the new getters.

Change-Id: I5101755e47657c25f10be1417f105e3ae72a3c39
Reviewed-on: https://chromium-review.googlesource.com/1126919
Commit-Queue: Théotime Grohens <theotime@google.com>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54271}