intel/compiler: rework conversion opcodes

intel/compiler: rework conversion opcodes

Now that we have the regioning lowering pass we can just put all of these
opcodes together in a single block and we can just assert on the few cases
of conversion instructions that are not supported in hardware and that should
be lowered in brw_nir_lower_conversions.

The only cases what we still handle separately are the conversions from float
to half-float since the rounding variants would need to fallthrough and we
are already doing this for boolean opcodes (since they need to negate), plus
there is also a large comment about these opcodes that we probably want to
keep so it is just easier to keep these separate.

Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>