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NIR: intel: Enhancements to nir_opt_peephole_select

Ian Romanick requested to merge idr/mesa:review/peephole_select into master

Generally, this makes the bcsel peephole optimization a little more greedy. The first patch causes the pass to not include things that are expected to become source or destination modifiers in the instructions. This causes more if-statement to be converted.

Later patches try to clean up some of the odd patterns that may result from lowering if-statements to a series of bcsel. The latter was not as successful as I would have liked.

I think we may want to either modify this pass or add a separate pass to do some code motion like this. I see a lot of shaders that do things like:

    if (condition) {
        // some math
        coord = ...;
        x = texture(s, coord);
    } else {
        // some different math
        coord = ...;
        x = texture(s, coord);
    }

If the condition is non-uniform, this is not ideal. A pass like this could move the texture access out of the if-statement.

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