The NIR story on conversion opcodes is a mess.  We've had way too many
of them, naming is inconsistent, and which ones have explicit sizes was
sort-of random.  This commit re-organizes things and makes them all
consistent:

 - All non-bool conversion opcodes now have the explicit size in the
   destination and are named <src_type>2<dst_type><size>.

 - Integer <-> integer conversion opcodes now only come in i2i and u2u
   forms (i2u and u2i have been removed) since the only difference
   between the different integer conversions is whether or not they
   sign-extend when up-converting.

 - Boolean conversion opcodes all have the explicit size on the bool and
   are named <src_type>2<dst_type>.

Making things consistent also allows nir_type_conversion_op to be moved
to nir_opcodes.c and auto-generated using mako.  This will make adding
int8, int16, and float16 versions much easier when the time comes.

Reviewed-by: Eric Anholt <eric@anholt.net>