This CL adds const-eval for `tan` and `tanh`.
Bug: tint:1581
Change-Id: I3d3506a6e7462bba1557cb88065d696ddc21b0f6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/109562
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>