This ensures mathematically correct results on large inputs.

BUG=chromium:795269
BUG=angleproject:1093
TEST=dEQP-GLES3.functional.shaders.builtin_functions.precision.tanh*

Change-Id: Id5ba05a3284e51a34f196b419abef0f4a41551e0
Reviewed-on: https://chromium-review.googlesource.com/832463Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>