compiler: replace MAYBE_UNUSED with UNUSED

MAYBE_UNUSED is going away, so let's replace legitimate uses of it with
UNUSED, which the former aliased to so far anyway.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/nir/nir_search.c

@@ -549,7 +549,7 @@ construct_value(nir_builder *build,
    }
 }
 
-MAYBE_UNUSED static void dump_value(const nir_search_value *val)
+UNUSED static void dump_value(const nir_search_value *val)
 {
    switch (val->type) {
    case nir_search_value_constant: {