nir: Make nir_constant a vector rather than a matrix

Merged Faith Ekstrand requested to merge gfxstrand/mesa:review/nir-constant-rework into master

Most places in NIR, we treat matrices like arrays. The one annoying exception to this has been nir_constant where a matrix is a first-class thing. This MR changes that so a matrix nir_constant is the same as an array nir_constant. This makes matrix nir_constants a tiny bit more expensive but shrinks all others by 96B.

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