i965/fs_inst: Add an is_copy_payload helper

i965/fs_inst: Add an is_copy_payload helper

This commit adds a new is_copy_payload helper to fs_inst that takes the
place of the similarly named functions in cse and register coalesce.  The
two is_copy_payload functions in CSE and register coalesce were subtly
different and potentially subtly broken.  The new version unifies the two
and should be more correct.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>