[enhancement] Make invoke side-effecting + make a "safe to remove side-effecting op" category

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

There are places where we need to decide if it's OK to remove or change an op, and the check often looks something like

```
if (OpIsSideEffecting(node->op()) && !node->Is()) { ... }
```

or something like that. Relatedly, #1805 implies that perhaps `invoke()` ops should be considered side-effecting, but you probably should be able to remove ffi-invokes.

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

Ad-hoc categories scattered throughout code + hope that fuzzing catches when these categories are misaligned.

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

Make a category like `MustKeepSideEffectOp()` that excludes `gate()` and FFI `invokes()`, but includes "regular" invokes. Alternatively, make a separate op for FFI stuff.