This requires a pretty significant rework of encode_alu_base(). In
particular, we can't know the register file that's going to be used
until we get into encode_alu_base() so ALUSrc::from_src() can't handle
Zero itself. Instead, we defer to a new ALUSrc::with_op_uniformity()
helper which does a postprocess step.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29591>
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