mix of ivec is not core until later GLSL versions.
Just polyfill in the writer with ternary ops.
Change-Id: Ia0c35bf95842e03ef8447019f3264d01c11fd384
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/123240
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Kokoro: Ben Clayton <bclayton@google.com>
