nir: add nir_def_block helper

Another common composition.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Acked-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36489>

src/compiler/nir/nir.h

@@ -1019,6 +1019,12 @@ typedef struct nir_def {
    bool loop_invariant;
 } nir_def;
 
+static inline nir_block *
+nir_def_block(nir_def *def)
+{
+   return def->parent_instr->block;
+}
+
 typedef struct nir_src {
    /* Instruction or if-statement that consumes this value as a source. This
     * should only be accessed through nir_src_* helpers.