Only test precision in compute stages

Only test precision in compute stages

These functions are unlikely to be implemented differently in different
shader stages, so only test them in a single stage. This makes the code
quite a bit simpler.

This change also removes a significant amount of duplicated code and
code that is never used.

Component: Vulkan
Affects: dEQP-VK.glsl.builtin.precision*
Tests removed: dEQP-VK.glsl.builtin.precision*vertex.*
               dEQP-VK.glsl.builtin.precision*fragment.*

Change-Id: Id3d9a17b16796e70e20703fffe6dd8c0f2fecfd1