intel/compiler: Add ctor to fs_builder that just takes the shader

Uses the dispatch_width from the shader (fs_visitor).  This was not
possible before because the dispatch_width was not part of
backend_shader.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26323>

src/intel/compiler/test_fs_saturate_propagation.cpp

@@ -72,7 +72,7 @@ saturate_propagation_test::saturate_propagation_test()
 
    v = new saturate_propagation_fs_visitor(compiler, &params, prog_data, shader);
 
-   bld = fs_builder(v, v->dispatch_width).at_end();
+   bld = fs_builder(v).at_end();
 
    devinfo->ver = 6;
    devinfo->verx10 = devinfo->ver * 10;