Support SameAsFirstInput policy in register allocator when inputs are paired machine registers

runtime/vm/flow_graph_allocator.cc

Index: runtime/vm/flow_graph_allocator.cc
diff --git a/runtime/vm/flow_graph_allocator.cc b/runtime/vm/flow_graph_allocator.cc
index bb092dfc8c810bf52cb39c8c94129f9e5035f006..e5f0865d8bff0b89cfb3aee8f42c5db0488d555b 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -1150,9 +1150,20 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
   // Normalize same-as-first-input output if input is specified as
   // fixed register.
   if (locs->out(0).IsUnallocated() &&
-      (locs->out(0).policy() == Location::kSameAsFirstInput) &&
-      (locs->in(0).IsMachineRegister())) {
-    locs->set_out(0, locs->in(0));
+      (locs->out(0).policy() == Location::kSameAsFirstInput)) {
+    if (locs->in(0).IsPairLocation()) {
+      // Pair input, pair output.
+      PairLocation* in_pair = locs->in(0).AsPairLocation();
+      ASSERT(in_pair->At(0).IsMachineRegister() ==
+             in_pair->At(1).IsMachineRegister());
+      if (in_pair->At(0).IsMachineRegister() &&
+          in_pair->At(1).IsMachineRegister()) {
+        locs->set_out(0, Location::Pair(in_pair->At(0), in_pair->At(1)));
+      }
+    } else if (locs->in(0).IsMachineRegister()) {
+      // Single input, single output.
+      locs->set_out(0, locs->in(0));
+    }
   }
 
   const bool output_same_as_first_input =