i965/ir: Test thread dispatch packing assumptions.

i965/ir: Test thread dispatch packing assumptions.

Not [originally] intended for upstream.  Should cause a GPU hang if
some thread is executed with a non-contiguous dispatch mask breaking
assumptions of brw_stage_has_packed_dispatch().  Doesn't cause any
CTS, DEQP or Piglit regressions, while replacing
brw_stage_has_packed_dispatch() with a dummy implementation that
unconditionally returns true on top of this patch causes multiple GPU
hangs.

v2: Refactor into a separate function instead of emitting the test
    code directly from emit_nir_code(), drop VEC4 test and clean up
    slightly for upstream. (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>