intel/fs/cse: Make HALT instruction act as CSE barrier.

Found by inspection.  This seems particularly likely to cause problems
with instructions dependent on the current execution mask like
SHADER_OPCODE_FIND_LIVE_CHANNEL or the FS_OPCODE_LOAD_LIVE_CHANNELS
instruction I'm about to introduce, but one could imagine it leading
to data corruption if CSE ever managed to combine two instructions
before and after the FS_OPCODE_PLACEHOLDER_HALT, since the one before
may not be executed for some channels.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Cc: 20.0 <mesa-stable@lists.freedesktop.org>

src/intel/compiler/brw_fs_cse.cpp

@@ -317,6 +317,16 @@ fs_visitor::opt_cse_local(bblock_t *block, int &ip)
          }
       }
 
+      /* Discard jumps aren't represented in the CFG unfortunately, so we need
+       * to make sure that they behave as a CSE barrier, since we lack global
+       * dataflow information.  This is particularly likely to cause problems
+       * with instructions dependent on the current execution mask like
+       * SHADER_OPCODE_FIND_LIVE_CHANNEL.
+       */
+      if (inst->opcode == FS_OPCODE_DISCARD_JUMP ||
+          inst->opcode == FS_OPCODE_PLACEHOLDER_HALT)
+         aeb.make_empty();
+
       foreach_in_list_safe(aeb_entry, entry, &aeb) {
          /* Kill all AEB entries that write a different value to or read from
           * the flag register if we just wrote it.