pan/bi: Pull out bi_dontcare helper

Where we need a power efficient encoding but don't care about the value
read, corresponding to rNULL in canonical assembly.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Maciej Matuszczyk <maccraft123mc@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8427>

src/panfrost/bifrost/bifrost_compile.c

@@ -208,7 +208,7 @@ bi_emit_load_vary(bi_builder *b, nir_intrinsic_instr *instr)
         /* Ignored for non-conditional center and retrieve modes (use an
          * efficient encoding), otherwise R61 for sample mask XXX RA */
 
-        bi_index src0 = (sample == BI_SAMPLE_CENTER) ? bi_passthrough(BIFROST_SRC_FAU_HI) :
+        bi_index src0 = (sample == BI_SAMPLE_CENTER) ? bi_dontcare() :
                 bi_register(61);
 
         nir_src *offset = nir_get_io_offset_src(instr);

src/panfrost/bifrost/compiler.h

@@ -150,6 +150,13 @@ bi_passthrough(enum bifrost_packed_src value)
         };
 }
 
+/* Read back power-efficent garbage, TODO maybe merge with null? */
+static inline bi_index
+bi_dontcare()
+{
+        return bi_passthrough(BIFROST_SRC_FAU_HI);
+}
+
 /* Extracts a word from a vectored index */
 static inline bi_index
 bi_word(bi_index idx, unsigned component)