Spread the array zeroing across as many workgroup invocations as possible.
Bug: tint:910
Change-Id: I1cb5a6aaafd2a0a4093ea3b9797c173378bc5605
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60203
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>