nir: Switch the arguments to nir_foreach_def

This matches the "foreach x in container" pattern found in many other
programming languages.  Generated by the following regular expression:

s/nir_foreach_def(\([^,]*\),\s*\([^,]*\))/nir_foreach_def(\2, \1)/

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/nir/nir.h

@@ -546,10 +546,10 @@ nir_dest_init(void)
 
 #define NIR_DEST_INIT nir_dest_init()
 
-#define nir_foreach_def(reg, dest) \
+#define nir_foreach_def(dest, reg) \
    list_for_each_entry(nir_dest, dest, &(reg)->defs, reg.def_link)
 
-#define nir_foreach_def_safe(reg, dest) \
+#define nir_foreach_def_safe(dest, reg) \
    list_for_each_entry_safe(nir_dest, dest, &(reg)->defs, reg.def_link)
 
 static inline nir_src