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.
This makes two changes to how example exprs are selected. Example exprs
are now ordered separately by each piece of the location, rather than by
stringifying their location. Second, UnknownLocations are now ordered
after locations with absolute paths, by using "~" in the lexicographic
comparison of absolute paths. I think this works on both POSIX and
Windows systems, but it's possible I'm missing a way to start an
absolute path with a unicode character.
