Add NaN support to Float32ToFloat16.

This follows Dawn's implementation:
https://dawn-review.googlesource.com/c/dawn/+/8361

Bug: 3612
Test: angle_unittests
Change-Id: I624bee2fc48c46b8a69cd38654679ac3cbd7c09f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1678407
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>