Fix build with Python < 2.7.

  File "src/compiler/nir/nir_builder_opcodes_h.py", line 46, in <module>
    from nir_opcodes import opcodes
  File "src/compiler/nir/nir_opcodes.py", line 178, in <module>
    unop_convert("{}2{}{}".format(src_t[0], dst_t[0], bit_size),
ValueError: zero length field name in format

Fixes: 762a6333

 ("nir: Rework conversion opcodes")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>