Refactor stack IR / binary writer (NFC) (#2250)

Previously `StackWriter` and its subclasses had routines for all three
modes (`Binaryen2Binary`, `Binaryen2Stack`, and `Stack2Binary`) within a
single class. This splits routines for each in a separate class and
also factors out binary writing into a separate class
(`BinaryInstWriter`) so other classes can make use of it.

The new classes are:
- `BinaryInstWriter`:
   Binary instruction writer. Only responsible for emitting binary
   contents and no other logic
- `BinaryenIRWriter`: Converts binaryen IR into something else
  - `BinaryenIRToBinaryWriter`: Writes binaryen IR to binary
  - `StackIRGenerator`: Converts binaryen IR to stack IR
- `StackIRToBinaryWriter`: Writes stack IR to binary

Author: aheejin

build-js.sh

@@ -167,6 +167,7 @@ mkdir -p ${OUT}
   $BINARYEN_SRC/wasm/wasm-interpreter.cpp \
   $BINARYEN_SRC/wasm/wasm-io.cpp \
   $BINARYEN_SRC/wasm/wasm-s-parser.cpp \
+  $BINARYEN_SRC/wasm/wasm-stack.cpp \
   $BINARYEN_SRC/wasm/wasm-type.cpp \
   $BINARYEN_SRC/wasm/wasm-validator.cpp \
   $BINARYEN_SRC/wasm/wasm.cpp \

src/passes/StackIR.cpp

@@ -36,27 +36,10 @@ struct GenerateStackIR : public WalkerPass<PostWalker<GenerateStackIR>> {
   bool modifiesBinaryenIR() override { return false; }
 
   void doWalkFunction(Function* func) {
-    BufferWithRandomAccess buffer;
-    // a shim for the parent that a stackWriter expects - we don't need
-    // it to do anything, as we are just writing to Stack IR
-    struct Parent {
-      Module* module;
-      Parent(Module* module) : module(module) {}
-
-      Module* getModule() { return module; }
-      void writeDebugLocation(Expression* curr, Function* func) {
-        WASM_UNREACHABLE();
-      }
-      Index getFunctionIndex(Name name) { WASM_UNREACHABLE(); }
-      Index getFunctionTypeIndex(Name name) { WASM_UNREACHABLE(); }
-      Index getGlobalIndex(Name name) { WASM_UNREACHABLE(); }
-    } parent(getModule());
-    StackWriter<StackWriterMode::Binaryen2Stack, Parent> stackWriter(
-      parent, buffer, false);
-    stackWriter.setFunction(func);
-    stackWriter.visitPossibleBlockContents(func->body);
+    StackIRGenerator stackIRGen(getModule()->allocator, func);
+    stackIRGen.write();
     func->stackIR = make_unique<StackIR>();
-    func->stackIR->swap(stackWriter.stackIR);
+    func->stackIR->swap(stackIRGen.getStackIR());
   }
 };