pan/bi: Stub 64-bit in count_write_registers

When Valhall lands, we'll need to be more methodical about this. In the
mean time, this gets validation passing on
KHR-GLES31.core.compute_shader.atomic-case3 which was crashing in RA and
now again passes.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12130>

src/panfrost/bifrost/bir.c

@@ -105,6 +105,8 @@ bi_count_write_registers(const bi_instr *ins, unsigned d)
                         return 4;
                 else
                         return bi_count_staging_registers(ins);
+        } else if (ins->op == BI_OPCODE_SEG_ADD_I64) {
+                return 2;
         }
 
         return 1;