Fix bot breakage: math unary instruction has side effect and therefore cannot be eliminated

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 32381)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -1419,8 +1419,11 @@
 }
 
 
+// A math unary instruction has a side effect (exception
+// thrown) if the argument is not a number.
+// TODO(srdjan): eliminate if has no uses and input is guaranteed to be number.
 Definition* MathUnaryInstr::Canonicalize(FlowGraph* flow_graph) {
-  return HasUses() ? this : NULL;
+  return this;
 }