[turboshaft] fix single-block loop bug

Bug: v8:12783
Change-Id: Ia4aaf245428dd63be09a33bfb684f0573f6b9296
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3913084Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83420}

src/compiler/turboshaft/optimization-phase.h

@@ -290,12 +290,13 @@ struct OptimizationPhase<Analyzer, Assembler>::Impl {
 
   V8_INLINE OpIndex ReduceGoto(const GotoOp& op) {
     Block* destination = MapToNewGraph(op.destination->index());
+    assembler.current_block()->SetOrigin(current_input_block);
+    assembler.Goto(destination);
     if (destination->IsBound()) {
       DCHECK(destination->IsLoop());
       FixLoopPhis(destination);
     }
-    assembler.current_block()->SetOrigin(current_input_block);
-    return assembler.Goto(destination);
+    return OpIndex::Invalid();
   }
   V8_INLINE OpIndex ReduceBranch(const BranchOp& op) {
     Block* if_true = MapToNewGraph(op.if_true->index());