gallivm/nir: lower tg4 offsets.

Fixes:
dEQP-VK.glsl.texture_gather.offsets.*

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6381>

src/gallium/auxiliary/gallivm/lp_bld_nir.c

@@ -2016,6 +2016,12 @@ bool lp_build_nir_llvm(
 void lp_build_opt_nir(struct nir_shader *nir)
 {
    bool progress;
+
+   static const struct nir_lower_tex_options lower_tex_options = {
+      .lower_tg4_offsets = true,
+   };
+   NIR_PASS_V(nir, nir_lower_tex, &lower_tex_options);
+
    do {
       progress = false;
       NIR_PASS_V(nir, nir_opt_constant_folding);