[XLA:GPU] Don't fuse non-scalar constants in GPU executables.

Constants within fusions don't get the special treatment that top-level GPU
constants get that lets us avoid putting them in the IR.

We could fix the fusion emitter to be aware of this GPU-specific thing, but I
sort of think it's better to make the representation explicit by pulling the
constants out of the fusion.

PiperOrigin-RevId: 218285050