mesa/main: ff-fragshader to nir
There's little point in emitting GLSL IR for the fixed-function fragment shaders, when we can emit NIR directly instead.
This simplifies things a bit, and makes the fixed-function vertex and fragment shaders look a lot more alike.
This is the fragment-shader equivalent of !22520 (merged). Unlike the vertex-shader version, this only saves a modest amount of code. But we should still save some compile-time etc due to bypassing the GLSL-bits entirely.