intel/nir: fix potential invalid function impl ptr usage

We keep the nir_builder::impl value around, but we've run some passes
that might have change the main function.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 96fde5518b ("intel/rt: Add a helper to create the raygen trampoline shader")
Acked-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16970>

src/intel/compiler/brw_nir_rt.c

@@ -489,6 +489,7 @@ brw_nir_create_raygen_trampoline(const struct brw_compiler *compiler,
 
    NIR_PASS_V(nir, brw_nir_lower_rt_intrinsics, devinfo);
 
+   nir_builder_init(&b, nir_shader_get_entrypoint(b.shader));
    /* brw_nir_lower_rt_intrinsics will leave us with a btd_global_arg_addr
     * intrinsic which doesn't exist in compute shaders.  We also created one
     * above when we generated the BTD spawn intrinsic.  Now we go through and