[LVN] Missed constant fold

[cgyurgyik]

I was checking my implementation against the examples, and found this:
bril2json < overwritten-variable.bril | python3 examples/lvn.py -p -c -f | bril2txt

@main {
  v1: int = const 4;
  v2: int = const 0;
  mul1: int = mul v1 v2;
  v2: int = const 3;
}

Lowers to:

@main {
  v1: int = const 4;
  lvn.2: int = const 0;
  mul1: int = mul v1 lvn.2; // <-- This should be folded.
  v2: int = const 3;
}

Expected output:

@main {
  v1: int = const 4;
  lvn.2: int = const 0;
  mul1: int = const 0;
  v2: int = const 3;
}

[cgyurgyik]

Let me know if I'm wrong here. I'll come back to this later if so, I need a break from LVN for a little :-)

[sampsyo]

Huh, yeah, it does look like it should do that. I remember wrestling with something similar earlier, perhaps in the context of #77, but I don't really know why it's missing this.