[LoopInterchange] Incrementally update the dominator tree.

We can use incremental dominator tree updates to avoid re-calculating
the dominator tree after interchanging 2 loops.

Reviewers: dmgreen, kuhar

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D43176

llvm-svn: 325122
diff --git a/llvm/test/Transforms/LoopInterchange/phi-ordering.ll b/llvm/test/Transforms/LoopInterchange/phi-ordering.ll
index d2d2947..c24e6eb 100644
--- a/llvm/test/Transforms/LoopInterchange/phi-ordering.ll
+++ b/llvm/test/Transforms/LoopInterchange/phi-ordering.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -loop-interchange -S | FileCheck %s
+; RUN: opt < %s -loop-interchange -verify-dom-info -S | FileCheck %s
 ;; Checks the order of the inner phi nodes does not cause havoc.
 ;; The inner loop has a reduction into c. The IV is not the first phi.