intel/fs: Add support for float16 to the fsign optimizations

intel/fs: Add support for float16 to the fsign optimizations

Commit ad98fbc2174 ("intel/fs: Refactor code generation for nir_op_fsign
to its own function") criss-crossed with c2b8fb9a810 ("anv/device:
expose VK_KHR_shader_float16_int8 in gen8+"), and I was not paying
enough attention when I rebased.  This adds back the float16 changes and
enables the optimization.

v2: Incorporate more changes from 19cd2f5debd and a8d8b1a1391 that I
missed in the previous version.

Fixes: ad98fbc2174 ("intel/fs: Refactor code generation for nir_op_fsign to its own function")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110474
Reviewed-by: Matt Turner <mattst88@gmail.com> [v1]