fix return unreachable in BlockStatement causes validate failed (#2187)

Author: HerrCai0907

src/compiler.ts

@@ -2793,15 +2793,24 @@ export class Compiler extends DiagnosticEmitter {
     }
 
     // Otherwise emit a normal return
-    return !expr
-      ? isLastInBody
-        ? module.nop()
-        : module.return()
-      : isLastInBody
-        ? expr
-        : this.currentType == Type.void
-          ? module.block(null, [ expr, module.return() ])
-          : module.return(expr);
+    if (expr) {
+      if (getExpressionId(expr) == ExpressionId.Unreachable) {
+        // emit unreachable directly
+        return expr;
+      }
+      if (isLastInBody) {
+        return expr;
+      }
+      if (this.currentType == Type.void) {
+        return module.block(null, [ expr, module.return() ]);
+      }
+      return module.return(expr);
+    } else {
+      if (isLastInBody) {
+        return module.nop();
+      }
+      return module.return();
+    }
   }
 
   private compileSwitchStatement(

src/passes/shadowstack.ts

@@ -658,6 +658,7 @@ class InstrumentReturns extends Pass {
     var stmts = new Array<ExpressionRef>();
     if (value) {
       let returnType = _BinaryenExpressionGetType(value);
+      if (returnType != TypeRef.Unreachable) return;
       let temp = this.parentPass.getSharedTemp(this.currentFunction, returnType);
       // t = value
       stmts.push(