intel/fs: make scan/reduce work with SIMD32 when it fits 2 registers

intel/fs: make scan/reduce work with SIMD32 when it fits 2 registers

When dealing with uint16_t and uint8_t on SIMD32 we can do all the
operations using just 2 registers, so we don't hit the recursion at
the beginning of emit_scan(). Because of that, we need to actually
compute scan/reduce for channels 31:16.

v2: Still missed instructions (Jason).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>