Add stacky mode to BinaryenIR

[tlively]

As an alternative to StackIR, adds a "stacky" mode to the normal
BinaryenIR in which normal instructions are completely flat and have
only pops as their children. This means that all instructions in
blocks may have arbitrary concrete types, not just the last
instructions.

The potential benefits of introducing a new mode for Binaryen IR are
that many passes will be agnostic to whether they run on the stacky or
unstacky code and stacky optimizations might be easier to write. This
will require further comparative exploration.

The next steps in this direction are:

• Perform the actual tuple lowering in Stackify.cpp

• Add a new validation mode that checks invariants for stacky code

• Testing and fuzzing support

• Reimplement the StackIR optimizations on the stacky BinaryenIR

• Add checking to the pass runner to ensure passes get only code
  forms they can handle.

No more work will be done on this PR until tuples are made to work
with the current StackIR. Then maybe we can look more concretely at
the tradeoffs involved.

[kripken]

How would this handle the problems we discussed on the whiteboard a while ago about pops? IIRC the order of pops was potentially very confusing etc.

[tlively]

The difference here is that it is an all-or-nothing approach. Pops are only hard to reason about when they are mixed with normal nested IR. In this approach mixing nested IR and stacky IR is forbidden. My hope is that sharing IR data structures will make it much easier to make passes that are stackiness-agnostic, but I expect that most passes will still expect one form or the other, as they do today.

[kripken]

I see, thanks. Interesting!