Some conversions are not directly supported in hardware and need to be
split in two conversion instructions going through an intermediary type.
Doing this at the NIR level simplifies a bit the complexity in the backend.

v2:
 - Consider fp16 rounding conversion opcodes
 - Properly handle swizzles on conversion sources.

v3
 - Run the pass earlier, right after nir_opt_algebraic_late (Jason)
 - NIR alu output types already have the bit-size (Jason)
 - Use 'is_conversion' to identify conversion operations (Jason)

v4:
 - Be careful about the intermediate types we use so we don't lose
   range and avoid incorrect rounding semantics (Jason)

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