d3d12: overlapping-locations-input-attribs crash
The following two piglits crash on D3D12:
spec@arb_explicit_attrib_location@overlapping-locations-input-attribs apispec@arb_explicit_attrib_location@overlapping-locations-input-attribs shader
The reason is that these repeat the same location variables for the vertex-shader, and DXIL doesn't allow that.
So we'll need some sort of lowering code that merges overlapping vertex locations. There's something similar to this in nir_lower_io_to_vector, but it's not quite what we want either.