[FIRRTL] Canonicalize propassign/ref.define, like canonicalizeSingleSetConnect

Consider:
```
firrtl.circuit "WireProbe" {
firrtl.module @WireProbe(in %x: !firrtl.uint<5>, out %p: !firrtl.probe>) {
%0 = firrtl.ref.send %x : !firrtl.uint<5>
%w = firrtl.wire : !firrtl.probe>
firrtl.ref.define %w, %0 : !firrtl.probe>
firrtl.ref.define %p, %w : !firrtl.probe>
}
}
```

We should be able to forward through the wire and drop it, as happens with strictconnect of hw things.

Similarly for propassign.

This will help remove wires without doing constant-prop, such as wires used in a passthrough module.

Start by reading the existing canonicalizeSingleSetConnect behavior and tracing the FIRRTL propassign and ref.define canonicalization paths. Confirm that a wire can be forwarded through and removed, and apply the same expectation to propassign; the issue is done when both cases canonicalize without constant propagation.

Written by the indexing model from the issue text.