aco: fix nir_op_frexp_exp with 16-bit floats and negative exponents

aco: fix nir_op_frexp_exp with 16-bit floats and negative exponents

v_frexp_exp_i16_f16 returns the two's complement for negative
exponents. For example, with 0.333252 it returns 0.666504 for
the mantissa and 65535 for the exponent (-1 in decimal).

RADV/LLVM and AMDVLK do a v_bfe_i32 and AMDGPU-PRO uses SDWA with
the sign extension bit set. The latter is probably what we want to
do in long term but for now RA doesn't support changing non-SDWA
instructions to SDWA if useful/needed.

Fixes dEQP-VK.glsl.builtin.precision_fp16_storage16b.frexp.compute.*.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4546>