i965/fs: Take Dispatch/Vector mask into account in FIND_LIVE_CHANNEL

i965/fs: Take Dispatch/Vector mask into account in FIND_LIVE_CHANNEL

On at least Sky Lake, ce0 does not contain the full story as far as enabled
channels goes.  It is possible to have completely disabled channels where
the corresponding bits in ce0 are 1.  In order to get the correct execution
mask, you have to mask off those channels which were disabled from the
beginning by taking the AND of ce0 with either sr0.2 or sr0.3 depending on
the shader stage.  Failure to do so can result in FIND_LIVE_CHANNEL
returning a completely dead channel.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: Francisco Jerez <currojerez@riseup.net>
[ Francisco Jerez: Fix a couple of typos, add mask register type
  assertion, clarify reason why ce0 can have bits set for disabled
  channels, clarify that this may only be a problem when thread
  dispatch doesn't pack channels tightly in the SIMD thread.  Apply
  same treatment to Align16 path. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>