nir: Just return when asked to rewrite uses of an SSA def to itself.

nir: Just return when asked to rewrite uses of an SSA def to itself.

The nir_builder swizzling improvement to not emit extra MOVs resulted in
nir_lower_tex() trying to rewrite an SSA def to itself, triggering the
assert on all texturing in v3d.  There's no work to be done in this case,
so just stop asserting.

Fixes: 743700be1f58 ("nir/builder: Don't emit no-op swizzles")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>