[llvm] [NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient (PR #111514)

[David Sherwood via llvm-commits]

================
@@ -556,6 +556,26 @@ class VPBlockBase {
     return getEnclosingBlockWithPredecessors()->getSinglePredecessor();
   }
 
+  /// This function replaces one predecessor with another, useful when
+  /// trying to replace an old block in the CFG with a new one.
+  void replacePredecessor(VPBlockBase *Old, VPBlockBase *New) {
+    auto I = find(Predecessors, Old);
+    assert(I != Predecessors.end());
+    assert(Old->getParent() == New->getParent() &&
+           "replaced predecessor must have the same parent");
+    *I = New;
----------------
david-arm wrote:

For example, `removeSuccessor` does not leave the `Successor` argument in a valid state either because `this` will still be a predecessor of `Successor`. How about I make the functions private and then we can behave like `removeSuccessor`, unless you see value in these new interfaces being public?

https://github.com/llvm/llvm-project/pull/111514