i965/fs: Expose arbitrary channel execution groups to the IR.

i965/fs: Expose arbitrary channel execution groups to the IR.

This generalizes the current fs_inst::force_sechalf flag to allow
specifying channel enable groups other than 0 or 8.  At some point it
will likely make sense to fix the vec4 generator to support arbitrary
execution groups and then move the definition of fs_inst::group into
backend_instruction (e.g. so we can do FP64 in the VEC4 back-end).

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