[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);
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artagnon wrote:

```suggestion
  VPInstruction *VPI = new VPInstruction(0, {});
  VPBasicBlock *VPBB = Plan.createVPBasicBlock("VPBB1", I1);
```

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