SuperVectorization: fix 'isa' assertion

Supervectorization uses null pointers to SSA values as a means of communicating
the failure to vectorize.  In operation vectorization, all operations producing
the values of operation arguments must be vectorized for the given operation to
be vectorized.  The existing check verified if any of the value "def"
statements was vectorized instead, sometimes leading to assertions inside `isa`
called on a null pointer.  Fix this to check that all "def" statements were
vectorized.

PiperOrigin-RevId: 226941552