glsl: precision handling cleanups and special casing reduction
I've been trying out moving mediump lowering to NIR instead of GLSL. It's going to take some more work to see if it's viable, but here are cleanups I've found along the way. Only a tiny bit of shader-db scheduler noise in the first commit on freedreno, everything else stayed the same.
Also one lonely NIR mediump lowering fixup that shouldn't affect anything and doesn't deserve its own MR.