Wire up uniformity analysis and make uniform values scalars
Daniel Schurmann wrote a fantastic divergence analysis pass for nir: mesa/mesa!918 (merged). We need to hook it up, figure out how to plumb it through out-of-SSA (or make it work after out-of-SSA) and make IBC start emitting 1-wide instructions for anything that's not divergent. This will hopefully make a very nice dent in the number of SIMD16 programs we're able to emit vs. the old back-end.