I had missed that LDC actually uses vec4 units for its offset. This
means that we have to create a new instruction, and lower it in
ir3_nir_lower_io_offsets, similar to the existing SSBO instructions.
Unfortunately we can't assume that loads are always vec4-aligned, so we
have to use the alignment information that NIR gives us. Unfortunately,
it's currently woefully inadequate, and will have to be fixed to give us
good codegen in the future.

Part-of: <mesa!4568>