i965/fs: Split VGRFs after lowering pull constants

The split_virtual_grfs code doesn't properly rewrite reladdr so we need to
make sure that any uniform indirects are lowered away first.

This fixes the glsl-fs-uniform-indexed-by-swizzled-vec4.shader_test in piglit

Cc: "10.6" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -4780,11 +4780,11 @@ fs_visitor::optimize()
     */
    bld = fs_builder(this, 64);
 
-   split_virtual_grfs();
-
    assign_constant_locations();
    demote_pull_constants();
 
+   split_virtual_grfs();
+
 #define OPT(pass, args...) ({                                           \
       pass_num++;                                                       \
       bool this_progress = pass(args);                                  \