i965/fs: Return result of image atomic in a register of the expected type.

i965/fs: Return result of image atomic in a register of the expected type.

So the result is of float type if we're implementing the float
overload of imageAtomicExchange.  This is the only back-end change
required to support OES_shader_image_atomic AFAICT.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>