[VPlan] Fix unused variable warning

llvm-project/llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp:312:19: warning: unused variable 'EB' [-Wunused-variable]
  312 |     VPBasicBlock *EB = Plan.getExitBlocks().front();
      |                   ^~

This showed up in a non-assertions build.

Author: boomanaiden154

llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp

@@ -309,8 +309,8 @@ VPlanTransforms::introduceMasksAndLinearize(VPlan &Plan, bool FoldTail) {
   if (FoldTail) {
     assert(Plan.getExitBlocks().size() == 1 &&
            "only a single-exit block is supported currently");
-    VPBasicBlock *EB = Plan.getExitBlocks().front();
-    assert(EB->getSinglePredecessor() == Plan.getMiddleBlock() &&
+    assert(Plan.getExitBlocks().front()->getSinglePredecessor() ==
+               Plan.getMiddleBlock() &&
            "the exit block must have middle block as single predecessor");
 
     VPBuilder B(Plan.getMiddleBlock()->getTerminator());