[turbofan] Stand-alone deferred block splitting - full.

src/compiler/move-optimizer.cc

Index: src/compiler/move-optimizer.cc
diff --git a/src/compiler/move-optimizer.cc b/src/compiler/move-optimizer.cc
index b869185e60ed2aed9dbc939878d1874e92e4eda6..7c2bbe06b874005e74b3260fbcb8275e28df9978 100644
--- a/src/compiler/move-optimizer.cc
+++ b/src/compiler/move-optimizer.cc
@@ -59,6 +59,17 @@ void MoveOptimizer::Run() {
   }
   for (auto block : code()->instruction_blocks()) {
     if (block->PredecessorCount() <= 1) continue;
+    bool has_only_deferred = true;
+    for (RpoNumber pred_id : block->predecessors()) {
+      if (!code()->InstructionBlockAt(pred_id)->IsDeferred()) {
+        has_only_deferred = false;
+        break;
+      }
+    }
+    // This would pull down common moves. If the moves occur in deferred blocks,
+    // and the closest common successor is not deferred, we lose the
+    // optimization of just spilling/filling in deferred blocks.
+    if (has_only_deferred) continue;
     OptimizeMerge(block);
   }
   for (auto gap : to_finalize_) {