nvir/nir: use nir_lower_idiv

NIR provides a common implementation of this so we don't need to use a
hand-written built-in library.

v2:
- use idiv_precise instead

Especially important on SM70 where we don't have an assembler.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5377>

src/gallium/drivers/nouveau/codegen/nv50_ir_from_nir.cpp

@@ -3285,6 +3285,11 @@ Converter::run()
    NIR_PASS_V(nir, nir_lower_alu_to_scalar, NULL, NULL);
    NIR_PASS_V(nir, nir_lower_phis_to_scalar);
 
+   /*TODO: improve this lowering/optimisation loop so that we can use
+    *      nir_opt_idiv_const effectively before this.
+    */
+   NIR_PASS(progress, nir, nir_lower_idiv, nir_lower_idiv_precise);
+
    do {
       progress = false;
       NIR_PASS(progress, nir, nir_copy_prop);