Made `Node::getType()`, `Node::asParameter()`, and `Node::asUninitialized()` operate directly on the IR. This actually fixed several diffs compared to the AST dataflow, because `getType()` wasn't holding for nodes that weren't `Exprs`.
Made `Uninitialized` a `VariableInstruction`. This makes it consistent with `InitializeParameter`.
It turns out that when building aliased SSA IR, we were still keeping around the Phi instructions from unaliased SSA IR. These leftover instructions didn't show up in dumps because they were not assigned to a block. However, when dumping additional instruction properties, they would show up as a top-level node in the dump, without a label.
