i915g: Switch to using nir-to-tgsi.

Emma Anholt requested to merge anholt/mesa:i915g-ntt-2 into main
    This fixes ~10% of the GLES2 failures thanks to having a better compiler,
    though in some cases we get some new compile fails due to instr count or
    uniform count.  We still have to do NIR-to-TGSI because the NIR gallivm
    draw path isn't ready for non-native-integer NIR code, and st/mesa treats
    native-integer as a screen property instead of a stage property.

Depends on !11249 (closed), part of getting to !8044 (merged).

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