Intel's Mesa Vec4 Code Now Unconditionally Uses NIR

As of a change yesterday to Intel's i965 Mesa driver, the Vec4 back-end is unconditionally using NIR rather than GLSL IR with the option being removed.

With the NIR intermediate representation being more efficient and having matured enough, NIR is permanently used by the Vec4 code. With this commit and other code pushed out on Friday, a lot of old, now-useless code could be removed to clean-up the driver by several thousand lines of code.

The final GLSL IR vs. NIR details for this code-path can be found via this change that dropped the NIR shader compiler option.