For loops only support assignments or function calls for the continuing statement.
Fixed: tint:1064
Change-Id: I07065b2119e7b9f97ca7e46b1464fd72333ca429
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60212
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Ben Clayton