Draft: nouveau/nir: Use natural alignment

We used to request vec4 alignment for everything on the nir codepath, but this triggers an assertion failure since a0b82c24, which prohibits vec4 alignment on scalars. Since requiring vec4 alignment on scalars is a little silly anyway, this patch relaxes the alignment to naturally aligned and disables the problematic backend pass that was preventing this earlier.

Fixes about 30 crashing tests in piglit and deqp on kepler, including eg piglit/tests/spec/glsl-1.30/execution/fs-large-local-array.shader_test