aco: fix half_pi constant for 16-bit fsin/fcos

This worked because the optimizer didn't consider that the 16-bit
instruction would interpret the inline constant differently. This will
change in the next commit.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5245>
1 file changed