spirv: Use a simpler and more correct implementaiton of tanh()

spirv: Use a simpler and more correct implementaiton of tanh()

The new implementation is more correct because it clamps the incoming value
to 10 to avoid floating-point overflow.  It also uses a much reduced
version of the formula which only requires 1 exp() rather than 2.  This
fixes all of the dEQP-VK.glsl.builtin.precision.tanh.* tests.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "13.0" <mesa-dev@lists.freedesktop.org>
(cherry picked from commit da1c49171d0df185545cfbbd600e287f7c6160fa)