vtn: Make tanh implementation even stupider

vtn: Make tanh implementation even stupider

The dEQP "precision" test tries to verify that the reference functions

   float sinh(float a) { return ((exp(a) - exp(-a)) / 2); }
   float cosh(float a) { return ((exp(a) + exp(-a)) / 2); }
   float tanh(float a) { return (sinh(a) / cosh(a)); }

produce the same values as the built-ins.  We simplified away the
multiplication by 0.5 in the numerator and denominator, and apparently
this causes them not to match for exactly 1 out of 13,632 values.

So, put it back in, fixing the test, but making our code generation
(and precision?) worse.