Revert "glsl: Use a simpler formula for tanh"

This reverts commit 9807f502eb7a023be619a14119388b2a43271b0e.

The simplified formula doesn't pass the tanh dEQP tests when we lower
to fp16 math.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4054>
1 file changed