intel/fs: Add a helper for emitting scan operations

intel/fs: Add a helper for emitting scan operations

This commit adds a helper to the builder for emitting "scan" operations.
Given a binary operation #, a scan takes the vector [a0, a1, ..., aN]
and returns the vector [a0, a0 # a1, ..., a0 # a1 # ... # aN] where each
channel contains the combination of all previous channels.  The sequence
of instructions to perform the scan is fairly optimal; a 16-wide scan on
a 32-bit type is only 6 instructions.  The subgroup scan and reduction
operations will be implemented in terms of this.

Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>