Ignore life ranges with no uses for a values defined as fixed output.

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 bcc9e6e278ea4d12f639160669bbfccf6e0cd85b..5ae1cd6a0c001d2ceeeb1f555bb2fec755efc3e3 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -755,6 +755,10 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
     // that will be allocated for this output's live range.
     // Special case: fixed output followed by a fixed input last use.
     UsePosition* use = range->first_use();
+
+    // If the value has no uses we don't need to allocate it.
+    if (use == NULL) return;
+
     if (use->pos() == (pos + 1)) {
       // We have a use position on the parallel move.
       ASSERT(use->location_slot()->IsUnallocated());