Have HInstruction::StrictlyDominates compute strict dominance.

Change-Id: I3a4fa133268615fb4ce54a0bcb43e0c2458cc865
diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index aee2177..10c6014 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -472,7 +472,11 @@
   return true;
 }
 
-bool HInstruction::Dominates(HInstruction* other_instruction) const {
+bool HInstruction::StrictlyDominates(HInstruction* other_instruction) const {
+  if (other_instruction == this) {
+    // An instruction does not strictly dominate itself.
+    return false;
+  }
   HBasicBlock* block = GetBlock();
   HBasicBlock* other_block = other_instruction->GetBlock();
   if (block != other_block) {