Add comment and dcheck regarding SuperblockCloner edge case

Based on a comment in r.android.com/3365219.

Bug: 368984521
Test: art/test/testrunner/testrunner.py -b --host --64 --optimizing
Change-Id: Ic8b945ba418df380158456bfb295edfd1dc0ad01

1 files changed, 3 insertions, 0 deletions

diff --git a/compiler/optimizing/superblock_cloner.cc b/compiler/optimizing/superblock_cloner.cc
index 06459c0fb4..276d2246cb 100644
--- a/compiler/optimizing/superblock_cloner.cc
+++ b/compiler/optimizing/superblock_cloner.cc
@@ -887,6 +887,9 @@ bool SuperblockCloner::IsSubgraphClonable() const {
     DCHECK_EQ(exits.size(), 1u);
     HInstruction* value = live_out.first;
     if (!value->GetBlock()->Dominates(exits[0])) {
+      // This case can only happen when `value` is used in a catch phi, so the graph must contain a
+      // catch block.
+      DCHECK(graph_->HasTryCatch());
       return false;
     }
   }