intel/compiler: add new half-float register type for 3-src instructions

intel/compiler: add new half-float register type for 3-src instructions

This is available since gen8.

v2: restore previously existing assertion.

v3: don't use separate tables for gen7 and gen8, just assert that we
    don't use half-float before gen8 (Matt)

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com> (v1)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>