vc4: Implement some useful instruction selection

[anholt]

This paper describes an instruction selection algorithm that seems really nice. It would probably remove the need for vc4_qir_lower_uniforms.c and fix #85, and probably make #86 easier.

Describing patterns of NIR instructions to match to QPU instruction sequences seems messy. Descriptions like src/compiler/nir/nir_opt_algebraic.py would be nice, and maybe we could reuse the NIR matching support. We'd want to be able to have some intrinsics in the matching, for things like discard_if in #86, so the NIR matching would need to be extended.