The new spelling `smoothstep` was introduced in M102.
Fixed: tint:1483
Change-Id: Ia5e1401f8f09450a3a767b0bb975216bd85be8db
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93360
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>