intel/fs: Rework fs_inst::is_copy_payload() into multiple classification helpers.

intel/fs: Rework fs_inst::is_copy_payload() into multiple classification helpers.

This reworks the current fs_inst::is_copy_payload() method into a
number of classification helpers with well-defined semantics.  This
will be useful later on in order to optimize LOAD_PAYLOAD instructions
more aggressively in cases where we can determine it's safe to do so.

The closest equivalent of the present fs_inst::is_copy_payload()
method is the is_coalescing_payload() helper introduced here.

No functional nor shader-db changes.

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