i965: Consider scratch writes to have side effects.

We could do better by tracking scratch reads and writes.

Cc: 10.5 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88793
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/mesa/drivers/dri/i965/brw_shader.cpp

@@ -963,6 +963,7 @@ backend_instruction::has_side_effects() const
 {
    switch (opcode) {
    case SHADER_OPCODE_UNTYPED_ATOMIC:
+   case SHADER_OPCODE_GEN4_SCRATCH_WRITE:
    case SHADER_OPCODE_URB_WRITE_SIMD8:
    case FS_OPCODE_FB_WRITE:
       return true;