[loopy.match] Targeting reduction expression in an instruction

In the following kernel:
```python
knl = lp.make_kernel(
["{[i]: 0<=i<8}",
"{[j]: 8<=j<10}"],
"""
out = sum(i, x[i]) + sum(j, x[j])
""")
```

It seems that there's no way to prefetch `x` in the sum-reduce over `i`. The root of the problem seems to be that the IR does not allow tagging reduction expressions.