i965/fs: Allow specifying arbitrary quarter control to FIND_LIVE_CHANNEL.
i965/fs: Allow specifying arbitrary quarter control to FIND_LIVE_CHANNEL.

This makes FIND_LIVE_CHANNEL behave like a normal instruction for
non-zero quarter control.  On Gen8+ we just leave the quarter control
field of the emitted FBL instruction set to the default value so the
hardware applies the expected shift to the execution mask signals.  On
Gen7 we apply the offset manually by specifying a non-zero subregister
offset in the source region of the FBL instruction.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler