[Dominators] Facilitate updates to MachinePostDominatorTree

Summary:
Add getBase accessor so that underlying tree can be
manipulated in a similar manner to MachineDominatorTree.

Reviewers: kuhar, arsenm, hliao, nhaehnle

Reviewed By: kuhar

Subscribers: lkail, mgorny, hiraditya, wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77967

llvm/include/llvm/CodeGen/MachinePostDominators.h

@@ -33,6 +33,12 @@ class MachinePostDominatorTree : public MachineFunctionPass {
 
   MachinePostDominatorTree();
 
+  PostDomTreeT &getBase() {
+    if (!PDT)
+      PDT.reset(new PostDomTreeT());
+    return *PDT;
+  }
+
   FunctionPass *createMachinePostDominatorTreePass();
 
   const SmallVectorImpl<MachineBasicBlock *> &getRoots() const {