intel/fs: Prevent emission of IR instructions not aligned to their own execution...

intel/fs: Prevent emission of IR instructions not aligned to their own execution size.

This can occur during payload setup of SIMD-split send message
instructions, which can lead to the emission of header setup
instructions with a non-zero channel group and fixed SIMD width.  Such
instructions could end up using undefined channel enable signals
except they don't care since they're always marked force_writemask_all.

Not known to affect correctness of any workload at this point, but it
would be trivial to back-port to stable if something comes up.

Reported-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Sagar Ghuge <sagar.ghuge@intel.com>