Revert "[VPlan] Support cloning initial VPlan (NFC)."
This reverts commit 5fa985e751c8f890fff31e190473aeeb6f7a9fc5.
Revert as this seems to introduce a call to a pure virtual function on a
few configs, e.g.
https://lab.llvm.org/buildbot/#/builders/169/builds/11535
