[Dominators] Change getNode parameter type to const NodeT * (NFC).

DominatorTreeBase::getNode does not modify its parameter and this change
allows callers that only have access to const pointers to use it without
casting.

Reviewers: kuhar, dblaikie, chandlerc

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D48231

llvm-svn: 334892

llvm/include/llvm/Support/GenericDomTree.h

@@ -351,15 +351,17 @@ class DominatorTreeBase {
   /// block.  This is the same as using operator[] on this class.  The result
   /// may (but is not required to) be null for a forward (backwards)
   /// statically unreachable block.
-  DomTreeNodeBase<NodeT> *getNode(NodeT *BB) const {
+  DomTreeNodeBase<NodeT> *getNode(const NodeT *BB) const {
     auto I = DomTreeNodes.find(BB);
     if (I != DomTreeNodes.end())
       return I->second.get();
     return nullptr;
   }
 
   /// See getNode.
-  DomTreeNodeBase<NodeT> *operator[](NodeT *BB) const { return getNode(BB); }
+  DomTreeNodeBase<NodeT> *operator[](const NodeT *BB) const {
+    return getNode(BB);
+  }
 
   /// getRootNode - This returns the entry node for the CFG of the function.  If
   /// this tree represents the post-dominance relations for a function, however,

llvm/unittests/IR/DominatorTreeTest.cpp

@@ -776,7 +776,9 @@ TEST(DominatorTree, InsertFromUnreachable) {
   PDT.insertEdge(From, To);
   EXPECT_TRUE(PDT.verify());
   EXPECT_TRUE(PDT.getRoots().size() == 2);
-  EXPECT_NE(PDT.getNode(B.getOrAddBlock("5")), nullptr);
+  // Make sure we can use a const pointer with getNode.
+  const BasicBlock *BB5 = B.getOrAddBlock("5");
+  EXPECT_NE(PDT.getNode(BB5), nullptr);
 }
 
 TEST(DominatorTree, InsertMixed) {