This uses FXC compilation failure mitigation for _any_ vector index assignment that has a non-constant index. FXC can still fall over if the loop calls a function that performs the dynamic index.
Use some vector swizzle logic to avoid branches in the helper.
Fixed: tint:980
Change-Id: I2a759d88a7d884bc61b4631cf57feb4acc8178de
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/57882
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
3242d3e8c9