Recompute dominator tree after DCE.

bug:22031382

(cherry picked from commit 1f82ecc6a0c9f88d03d6d1a6d95eeb8707bd06c1)

Change-Id: I9a74edb185cb806045903dfe9695d9cc1a02e86b
diff --git a/compiler/optimizing/boolean_simplifier.cc b/compiler/optimizing/boolean_simplifier.cc
index 8100a29..daf7d67 100644
--- a/compiler/optimizing/boolean_simplifier.cc
+++ b/compiler/optimizing/boolean_simplifier.cc
@@ -141,6 +141,12 @@
   block->MergeWith(false_block);
   block->MergeWith(merge_block);
 
+  // No need to update any dominance information, as we are simplifying
+  // a simple diamond shape, where the join block is merged with the
+  // entry block. Any following blocks would have had the join block
+  // as a dominator, and `MergeWith` handles changing that to the
+  // entry block.
+
   // Remove the original condition if it is now unused.
   if (!if_condition->HasUses()) {
     if_condition->GetBlock()->RemoveInstructionOrPhi(if_condition);