[reg-alloc] Propagate control-flow hint after splits

Change-Id: I01199e24c4bb5d8496c13077342e03a41943cdbd
Bug: v8:9088
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1684074Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62481}

src/compiler/backend/register-allocator.cc

@@ -4559,6 +4559,14 @@ void LinearScanAllocator::SpillBetweenUntil(LiveRange* range,
 
     LiveRange* third_part =
         SplitBetween(second_part, split_start, third_part_end);
+    if (GetInstructionBlock(data()->code(), second_part->Start())
+            ->IsDeferred()) {
+      // Try to use the same register as before.
+      TRACE("Setting control flow hint for %d:%d to %s\n",
+            third_part->TopLevel()->vreg(), third_part->relative_id(),
+            RegisterName(range->controlflow_hint()));
+      third_part->set_controlflow_hint(range->controlflow_hint());
+    }
 
     AddToUnhandled(third_part);
     // This can happen, even if we checked for start < end above, as we fiddle