i915g: Switch to using nir-to-tgsi.
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).