UniformityAnalysis could handle more kinds of InstructionUniformity

[jayfoad]

UniformityAnalysis categorizes instructions with a fixed set of InstructionUniformity values: Default, AlwaysUniform and NeverUniform.

It would be nice it if could also handle more complex ways in which the uniformity of an instruction depends on the uniformity of its operands. For example, a lane-permute operation like llvm.amdgcn.permlane64 is uniform if either of its operands (the incoming value or the lane select) is uniform. Maybe targets could override a hook where they could implement arbitrary uniformity-propagation rules like this.

Cc @ssahasra

[ssahasra]

This looks like an easy extension. Can you please file an internal ticket?