glsl_to_nir: Fix NIR bit-size of ir_triop_bitfield_extract and ir_quadop_bitfield_insert

glsl_to_nir: Fix NIR bit-size of ir_triop_bitfield_extract and ir_quadop_bitfield_insert

Previously these would return result->bit_size of 32 even though the
type might have been int16_t or uint16_t.  This prevents many assertion
failures in "glsl: Use nir_type_convert instead of
nir_type_conversion_op" on zink.

Fixes: 5e922fbc160 ("glsl_to_nir: fix bitfield_extract with 16-bit operands")
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15121>