`lp.extract_subst` does not search for subexpressions of `Product` or `Sum`

Consider the kernel:
```python
import loopy as lp

knl = lp.make_einsum("k,ij,ij->i", arg_names=("a", "b", "c"))
knl = lp.extract_subst(knl, "tmp_hoist", "b[i, j] * c[i, j]")
```

raises the following exception:
```python
RuntimeError: no expressions matching 'b[i, j]*c[i, j]'
```

The expected behavior would be that `b[i,j] * c[i,j]` should have resulted in a match.