nir/scheduler: Move nir_scheduler to its own header

nir_schedule already has a struct for options which makes it more than
just a function declaration. Later patches intend to add more structs to
complement these options. In order to make the code easier to manage,
this moves the nir_scheduler-related parts out of nir.h to their own
header.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5953>

src/broadcom/compiler/vir.c

@@ -24,6 +24,7 @@
 #include "broadcom/common/v3d_device_info.h"
 #include "v3d_compiler.h"
 #include "util/u_prim.h"
+#include "compiler/nir/nir_schedule.h"
 
 int
 vir_get_nsrc(struct qinst *inst)