Folding _ + NaN => NaN can widen type None to a constant type, which leads to floating DeadValue nodes. This CL fixes this by removing the optimization. Alternatively, we should consider removing all nodes of type None in simplified lowering.

Bug: chromium:817225
Change-Id: I2a126b360d70d3626f8a3c5e73ac72dc980ac8b3
Reviewed-on: https://chromium-review.googlesource.com/946129Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51699}