glsl: Use a simpler formula for tanh

The formula we have used in the past is a trivial reduction from the
definition by simply multiplying both the numerator and denominator of the
formula by 2.  However, multiplying by e^x, you can further reduce it.
This allows us to get rid of one side of the clamp and two of exponential
functions which should make it faster.  The new formula still passes the
dEQP precision tests for tanh so it should be fine.

Reviewed-by: Roland Scheidegger <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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