spirv2dxil: Remove dead branches early during shader compilation

Apparently spiregg can produce not-taken branches that somehow try
to access textures that aren't actually declared. These branches
need to be trimmed before any texture lowering is done.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27531>

src/microsoft/spirv_to_dxil/dxil_spirv_nir.c

@@ -1173,6 +1173,8 @@ dxil_spirv_nir_passes(nir_shader *nir,
          }
          NIR_PASS(progress, nir, nir_lower_vars_to_ssa);
          NIR_PASS(progress, nir, nir_opt_algebraic);
+         NIR_PASS(progress, nir, nir_opt_dead_cf);
+         NIR_PASS(progress, nir, nir_opt_remove_phis);
       } while (progress);
    }