Directly emit Wasm instructions for module initializers.

Before, we created synthetic IR trees that we sent to the expression
builder. Now, we instead directly emit the appropriate Wasm
instructions.

This adds a bit of duplication of logic on how arrays are built,
but it removes the direct dependency of the `Emitter` on
`WasmExpressionBuilder`.

Author: sjrd

Diff for: wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/Emitter.scala

@@ -210,27 +210,30 @@ final class Emitter(config: Emitter.Config) {
         instrs += WasmInstr.CALL(functionName)
       }
 
-      val stringArrayTypeRef = IRTypes.ArrayTypeRef(IRTypes.ClassRef(BoxedStringClass), 1)
-
-      val callTree = ModuleInitializerImpl.fromInitializer(init) match {
+      ModuleInitializerImpl.fromInitializer(init) match {
         case ModuleInitializerImpl.MainMethodWithArgs(className, encodedMainMethodName, args) =>
-          IRTrees.ApplyStatic(
-            IRTrees.ApplyFlags.empty,
-            className,
-            IRTrees.MethodIdent(encodedMainMethodName),
-            List(IRTrees.ArrayValue(stringArrayTypeRef, args.map(IRTrees.StringLiteral(_))))
-          )(IRTypes.NoType)
+          // vtable of Array[String]
+          instrs += GLOBAL_GET(genGlobalName.forVTable(BoxedStringClass))
+          instrs += I32_CONST(1)
+          instrs += CALL(genFunctionName.arrayTypeData)
+
+          // itable of Array[String]
+          instrs += GLOBAL_GET(genGlobalName.arrayClassITable)
+
+          // underlying array of args
+          args.foreach(arg => instrs ++= ctx.getConstantStringInstr(arg))
+          instrs += ARRAY_NEW_FIXED(genTypeName.anyArray, args.size)
+
+          // array object
+          val stringArrayTypeRef = IRTypes.ArrayTypeRef(IRTypes.ClassRef(BoxedStringClass), 1)
+          instrs += STRUCT_NEW(genTypeName.forArrayClass(stringArrayTypeRef))
+
+          // call
+          genCallStatic(className, encodedMainMethodName)
 
         case ModuleInitializerImpl.VoidMainMethod(className, encodedMainMethodName) =>
-          IRTrees.ApplyStatic(
-            IRTrees.ApplyFlags.empty,
-            className,
-            IRTrees.MethodIdent(encodedMainMethodName),
-            Nil
-          )(IRTypes.NoType)
+          genCallStatic(className, encodedMainMethodName)
       }
-
-      WasmExpressionBuilder.generateIRBody(callTree, IRTypes.NoType)
     }
 
     // Finish the start function