aco: fix 64-bit trunc with negative exponents on GFX6

aco: fix 64-bit trunc with negative exponents on GFX6

v_frexp_exp returns the exponent as an unsigned value.

Also, v_ashr returns either 0 or -1 depending on the sign of the
source operand, but what we want is only the sign bit.

Fixes a bunch of recent dEQP-VK.glsl.builtin.precision_double.* tests.

Cc: <mesa-stable@lists.freedesktop.org>
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/4921>