[enhancement] Validator that checks no nodes are dead out of the DSLX frontend IR conversion

### What's hard to do? (limit 100 words)

We noticed there are sometimes literals that are there for unclear reasons, e.g. literal.3 here:

```
top proc __test_module__P_0_next(__token: token, __state: bits[32], init={42}) {
literal.5: bits[32] = literal(value=1, id=5)
send.4: token = send(__token, __state, channel=test_module__s, id=4)
literal.3: bits[1] = literal(value=1, id=3)
new_state: bits[32] = add(__state, literal.5, id=6)
after_all.7: token = after_all(__token, send.4, id=7)
next (after_all.7, new_state)
}
```

It suggests maybe we could have a validator after IR conversion that checked there weren't dead non-side-effecting nodes just to make sure our DSLX-to-IR converter is spitting out IR without lots of junk in it.

### Current best alternative workaround (limit 100 words)

Just living with the junk nodes, they'll get DCE'd anyway.

### Your view of the "best case XLS enhancement" (limit 100 words)

We add the validator as described and fix up any code paths that flag validation issues.