nir: Add a halt instruction type

Halt is like a return for the entire shader or exit() if you prefer to
think of it that way.  Once an invocation hits a halt, it's 100% dead.
Any writes to output variables which happened before the halt do,
however, still apply.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7356>

src/compiler/nir/nir_validate.c

@@ -906,6 +906,7 @@ validate_jump_instr(nir_jump_instr *instr, validate_state *state)
 
    switch (instr->type) {
    case nir_jump_return:
+   case nir_jump_halt:
       validate_assert(state, block->successors[0] == state->impl->end_block);
       validate_assert(state, block->successors[1] == NULL);
       validate_assert(state, instr->target == NULL);