This CL adds const-eval for the `fma` builtin.
Bug: tint:1581
Change-Id: Ia4df818fec9d5d969b364b2c165400d787a9e275
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/111584
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>