i965/fs: handle fp64 opcodes in brw_do_channel_expressions

i965/fs: handle fp64 opcodes in brw_do_channel_expressions

In the case of the pack opcode we are already doing the
lowering in NIR, so no need to do it here. The unpack opcode
operates on scalars, so it should not be lowered.

In the case of frexp_sig and frexp_exp, they are lowered in
lower_instructions, so we don't have to care about them.

All the remaining opcodes involve conversions from and to doubles
and are business as usual.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>