On some Intel drivers, calling function isnan() on highp float will get wrong
answer. This patch work arounds this bug by using an expression to emulate
this function.

BUG=chromium:650547

Change-Id: I5bc5e0352c434f42cd2c55103a74f9f7ba51a72c
Reviewed-on: https://chromium-review.googlesource.com/389834
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>