i965/ir: Skip eliminate_find_live_channel() for stages with sparse thread dispatch.

i965/ir: Skip eliminate_find_live_channel() for stages with sparse thread dispatch.

The eliminate_find_live_channel optimization eliminates
FIND_LIVE_CHANNEL instructions in cases where control flow is known to
be uniform, and replaces them with 'MOV 0', which in turn unblocks
subsequent elimination of the BROADCAST instruction frequently used on
the result of FIND_LIVE_CHANNEL.  This is however not correct in
per-sample fragment shader dispatch because the PSD can dispatch a
fully unlit sample under certain conditions.  Disable the optimization
in that case.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
v2: Add devinfo argument to brw_stage_has_packed_dispatch() to
    implement hardware generation check.