i965/fs: Fix nir_op_fsign of absolute value.

i965/fs: Fix nir_op_fsign of absolute value.

This does point at the front-end emitting silly code that could have
been optimized out, but the current fsign implementation would emit
bogus IR if abs was set for the argument (because it would apply the
abs modifier on an unsigned integer type), and we shouldn't rely on
the upper layer's optimization passes for correctness.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>