Until now the execution controls (e.g. channel group,
force_writemask_all, exec_size) of the instruction had been completely
ignored by spilling, even though that can lead to a mismatch between
the channel mask applied to the contents of the (un)spilled memory and
the GRF source or destination of the instruction.  In some cases we'll
actually want the (un)spill messages to be marked force_writemask_all
regardless of whether the instruction has it set, but that will have
to be handled specially by the caller.

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