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/brw_fs_nir.cpp

@@ -2219,7 +2219,7 @@ fs_visitor::emit_gs_control_data_bits(const fs_reg &vertex_count)
 
    struct brw_gs_prog_data *gs_prog_data = brw_gs_prog_data(prog_data);
 
-   const fs_builder bld = fs_builder(this, dispatch_width).at_end();
+   const fs_builder bld = fs_builder(this).at_end();
    const fs_builder abld = bld.annotate("emit control data bits");
    const fs_builder fwa_bld = bld.exec_all();
 
@@ -8470,7 +8470,7 @@ fs_visitor::emit_nir_code()
    ntb->s = this;
    ntb->devinfo = devinfo;
    ntb->nir = nir;
-   ntb->bld = fs_builder(this, this->dispatch_width).at_end();
+   ntb->bld = fs_builder(this).at_end();
 
    emit_shader_float_controls_execution_mode(ntb);