[llvm] [VPlan] Move splitAt test to unit test. NFC (PR #164636)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -704,6 +704,18 @@ TEST_F(VPBasicBlockTest, reassociateBlocks) {
   }
 }
 
+TEST_F(VPBasicBlockTest, splitAtEnd) {
+  VPlan &Plan = getPlan();
+  VPInstruction *I1 = new VPInstruction(0, {});
+  VPBasicBlock *VPBB1 = Plan.createVPBasicBlock("VPBB1", I1);
+  VPBlockUtils::connectBlocks(Plan.getEntry(), VPBB1);
+  VPBlockUtils::connectBlocks(VPBB1, Plan.getScalarHeader());
+  VPBB1->splitAt(VPBB1->end());
+  auto *Split = cast<VPBasicBlock>(VPBB1->getSingleSuccessor());
+  EXPECT_TRUE(Split->empty());
----------------
artagnon wrote:

could also check VPBB1?

https://github.com/llvm/llvm-project/pull/164636