Don't mark register constraints resolution moves as prefering 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 9b0fe528c122fad54ddcf3b1f9102a97d5e45d3f..52a578ec7fec53f5e282800700530619b075e2b2 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -701,7 +701,7 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
       //      register   [-----)
       //
       MoveOperands* move =
-          AddMoveAt(pos - 1, *in_ref, Location::PrefersRegister());
+          AddMoveAt(pos - 1, *in_ref, Location::Any());
       BlockLocation(*in_ref, pos - 1, pos + 1);
       range->AddUseInterval(block->start_pos(), pos - 1);
       range->AddUse(pos - 1, move->src_slot());
@@ -821,7 +821,7 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
     range->DefineAt(pos + 1);
     if (range->Start() == range->End()) return;
 
-    MoveOperands* move = AddMoveAt(pos + 1, Location::PrefersRegister(), *out);
+    MoveOperands* move = AddMoveAt(pos + 1, Location::Any(), *out);
     range->AddUse(pos + 1, move->dest_slot());
   } else if (output_same_as_first_input) {
     // Output register will contain a value of the first input at instruction's
@@ -837,7 +837,7 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
     locs->set_out(Location::RequiresRegister());
     MoveOperands* move = AddMoveAt(pos,
                                    Location::RequiresRegister(),
-                                   Location::PrefersRegister());
+                                   Location::Any());
 
     // Add uses to the live range of the input.
     Value* input = current->InputAt(0);