[VPlan][Coverity] Fix coverity CID1579964. (llvm#121805)

Fix for the Coverity hit with CID1579964 in VPlan.cpp.

Coverity message with some context follows.

[Cov] var_compare_op: Comparing TermBr to null implies that TermBr might
be null.
434    } else if (TermBr && !TermBr->isConditional()) {
435      TermBr->setSuccessor(0, NewBB);
436    } else {
437 // Set each forward successor here when it is created, excluding
438 // backedges. A backward successor is set when the branch is
created.
439      unsigned idx = PredVPSuccessors.front() == this ? 0 : 1;
     	
[Cov] CID 1579964: (#1 of 1): Dereference after null check
(FORWARD_NULL)
[Cov] var_deref_model: Passing null pointer TermBr to getSuccessor,
which dereferences it.

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -438,10 +438,10 @@ void VPBasicBlock::connectToPredecessors(VPTransformState::CFGState &CFG) {
       // Set each forward successor here when it is created, excluding
       // backedges. A backward successor is set when the branch is created.
       unsigned idx = PredVPSuccessors.front() == this ? 0 : 1;
-      assert(
-          (!TermBr->getSuccessor(idx) ||
-           (isa<VPIRBasicBlock>(this) && TermBr->getSuccessor(idx) == NewBB)) &&
-          "Trying to reset an existing successor block.");
+      assert((TermBr && (!TermBr->getSuccessor(idx) ||
+                         (isa<VPIRBasicBlock>(this) &&
+                          TermBr->getSuccessor(idx) == NewBB))) &&
+             "Trying to reset an existing successor block.");
       TermBr->setSuccessor(idx, NewBB);
     }
     CFG.DTU.applyUpdates({{DominatorTree::Insert, PredBB, NewBB}});