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

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>