VM: Don't rely on always running constant propagation in the optimizer

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index 4c7cc958c015a69bb1839daa7f0db359bdad9ef2..84ffb0ebd1c7ecb4db4929e1af1679d66ee0c8ea 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -548,6 +548,10 @@ void InlineExitCollector::ReplaceCall(TargetEntryInstr* callee_entry) {
     call_->previous()->AppendInstruction(branch);
     call_block->set_last_instruction(branch);
 
+    // Replace uses of the return value with null to maintain valid
+    // SSA form - even though the rest of the caller is unreachable.
+    call_->ReplaceUsesWith(caller_graph_->constant_null());
+
     // Update dominator tree.
     call_block->AddDominatedBlock(callee_entry);
     call_block->AddDominatedBlock(false_block);