This CL adds const-eval for `smoothstep`. Bug: tint:1581 Change-Id: I78aa5c4a39882f29ff78e37313e6c44708719095 Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/110176 Reviewed-by: Antonio Maiorano <amaiorano@google.com> Kokoro: Kokoro <noreply+kokoro@google.com> Commit-Queue: Dan Sinclair <dsinclair@chromium.org>