intel/compiler: handle extended math restrictions for half-float

intel/compiler: handle extended math restrictions for half-float

Extended math with half-float operands is only supported since gen9,
but it is limited to SIMD8. In gen8 we lower it to 32-bit.

v2: quashed together the following patches (Jason):
  - intel/compiler: allow extended math functions with HF operands
  - intel/compiler: lower 16-bit extended math to 32-bit prior to gen9
  - intel/compiler: extended Math is limited to SIMD8 on half-float

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
  (allow extended math functions with HF operands,
   extended Math is limited to SIMD8 on half-float)