Stackify pass

Adds a Stackify ("--stackify") pass for converting normal Binaryen IR to Poppy
IR. Like the existing construction of Stacky IR, Stackify depends on the
BinaryenIRWriter to drive the emitting of instructions in correct stack machine
order. As instructions are "emitted," Stackify replaces their children with pops
and collects them in a list. At the end of each scope, Stackify creates a block
containing all the collected instructions for that scope and injects that block
into the enclosing scope. All tuple globals and instructions dealing with tuples
are also expanded to remove all tuples from the program.

The validator currently fails to validate many valid Poppy IR patterns produced
in the tests, but fixing that is left as follow-on work to keep this PR focused
on the Stackify pass itself. For now the tests simply skip validation.

Author: tlively

src/pass.h

@@ -171,6 +171,12 @@ struct PassRunner {
   PassRunner(const PassRunner&) = delete;
   PassRunner& operator=(const PassRunner&) = delete;
 
+  // But we can make it easy to create a nested runner
+  // TODO: Go through and use this in more places
+  explicit PassRunner(const PassRunner* runner)
+    : wasm(runner->wasm), allocator(runner->allocator),
+      options(runner->options), isNested(true) {}
+
   void setDebug(bool debug) {
     options.debug = debug;
     // validate everything by default if debugging

src/passes/CMakeLists.txt

@@ -57,6 +57,7 @@ set(passes_SOURCES
   PrintFeatures.cpp
   PrintFunctionMap.cpp
   RoundTrip.cpp
+  Stackify.cpp
   StackIR.cpp
   Strip.cpp
   StripTargetFeatures.cpp